BUG #16527: Valgrind detects an invalid read in brin_revmap_data with non-index page

The following bug has been logged on the website:

Bug reference:      16527
Logged by:          Alexander Lakhin
Email address:      exclusion@gmail.com
PostgreSQL version: 13beta2
Operating system:   Ubuntu 20.04
Description:

Running the following query (based on contrib/pageinspect/sql/brin.sql)
under valgrind:
CREATE EXTENSION pageinspect;
CREATE TABLE test1 (a int, b text);
INSERT INTO test1 VALUES (1, 'one');
SELECT * FROM brin_revmap_data(get_raw_page('test1', 0));

leads to a memory access error:
==00:00:00:12.518 934833== Invalid read of size 2
==00:00:00:12.518 934833==    at 0x4865AE1: verify_brin_page
(brinfuncs.c:107)
==00:00:00:12.518 934833==    by 0x486674E: brin_revmap_data
(brinfuncs.c:386)
==00:00:00:12.518 934833==    by 0x3C9656: ExecMakeTableFunctionResult
(execSRF.c:234)
==00:00:00:12.518 934833==    by 0x3DB7D4: FunctionNext
(nodeFunctionscan.c:95)
==00:00:00:12.518 934833==    by 0x3CA059: ExecScanFetch (execScan.c:133)
==00:00:00:12.518 934833==    by 0x3CA0F4: ExecScan (execScan.c:182)
==00:00:00:12.518 934833==    by 0x3DB6DF: ExecFunctionScan
(nodeFunctionscan.c:270)
==00:00:00:12.518 934833==    by 0x3C70B2: ExecProcNodeFirst
(execProcnode.c:450)
==00:00:00:12.518 934833==    by 0x3BFDD3: ExecProcNode (executor.h:245)
==00:00:00:12.518 934833==    by 0x3BFDD3: ExecutePlan (execMain.c:1646)
==00:00:00:12.518 934833==    by 0x3BFFB3: standard_ExecutorRun
(execMain.c:364)
==00:00:00:12.518 934833==    by 0x3C007F: ExecutorRun (execMain.c:308)
==00:00:00:12.518 934833==    by 0x55F21F: PortalRunSelect (pquery.c:912)
==00:00:00:12.518 934833==  Address 0xe69cc0a is 2 bytes after a block of
size 8,264 alloc'd
==00:00:00:12.518 934833==    at 0x483B7F3: malloc (in
/usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
==00:00:00:12.518 934833==    by 0x6A94CE: AllocSetAlloc (aset.c:739)
==00:00:00:12.518 934833==    by 0x6B2AA7: palloc (mcxt.c:963)
==00:00:00:12.518 934833==    by 0x486B838: get_raw_page_internal
(rawpage.c:154)
==00:00:00:12.518 934833==    by 0x486BC35: get_raw_page (rawpage.c:62)
==00:00:00:12.518 934833==    by 0x3BACBE: ExecInterpExpr
(execExprInterp.c:699)
==00:00:00:12.518 934833==    by 0x3B7A64: ExecInterpExprStillValid
(execExprInterp.c:1802)
==00:00:00:12.518 934833==    by 0x3C8C3B: ExecEvalExpr (executor.h:294)
==00:00:00:12.518 934833==    by 0x3C8C3B: ExecEvalFuncArgs
(execSRF.c:836)
==00:00:00:12.518 934833==    by 0x3C95C8: ExecMakeTableFunctionResult
(execSRF.c:181)
==00:00:00:12.518 934833==    by 0x3DB7D4: FunctionNext
(nodeFunctionscan.c:95)
==00:00:00:12.518 934833==    by 0x3CA059: ExecScanFetch (execScan.c:133)
==00:00:00:12.518 934833==    by 0x3CA0F4: ExecScan (execScan.c:182)
==00:00:00:12.518 934833== 
{
   <insert_a_suppression_name_here>
   Memcheck:Addr2
   fun:verify_brin_page
   fun:brin_revmap_data
   fun:ExecMakeTableFunctionResult
   fun:FunctionNext
   fun:ExecScanFetch
   fun:ExecScan
   fun:ExecFunctionScan
   fun:ExecProcNodeFirst
   fun:ExecProcNode
   fun:ExecutePlan
   fun:standard_ExecutorRun
   fun:ExecutorRun
   fun:PortalRunSelect
}
==00:00:00:12.519 934833== Invalid read of size 2
==00:00:00:12.519 934833==    at 0x4865C07: verify_brin_page
(brinfuncs.c:108)
==00:00:00:12.519 934833==    by 0x486674E: brin_revmap_data
(brinfuncs.c:386)
==00:00:00:12.519 934833==    by 0x3C9656: ExecMakeTableFunctionResult
(execSRF.c:234)
==00:00:00:12.519 934833==    by 0x3DB7D4: FunctionNext
(nodeFunctionscan.c:95)
==00:00:00:12.519 934833==    by 0x3CA059: ExecScanFetch (execScan.c:133)
==00:00:00:12.519 934833==    by 0x3CA0F4: ExecScan (execScan.c:182)
==00:00:00:12.519 934833==    by 0x3DB6DF: ExecFunctionScan
(nodeFunctionscan.c:270)
==00:00:00:12.519 934833==    by 0x3C70B2: ExecProcNodeFirst
(execProcnode.c:450)
==00:00:00:12.519 934833==    by 0x3BFDD3: ExecProcNode (executor.h:245)
==00:00:00:12.519 934833==    by 0x3BFDD3: ExecutePlan (execMain.c:1646)
==00:00:00:12.519 934833==    by 0x3BFFB3: standard_ExecutorRun
(execMain.c:364)
==00:00:00:12.519 934833==    by 0x3C007F: ExecutorRun (execMain.c:308)
==00:00:00:12.519 934833==    by 0x55F21F: PortalRunSelect (pquery.c:912)
==00:00:00:12.519 934833==  Address 0xe69cc0a is 2 bytes after a block of
size 8,264 alloc'd
==00:00:00:12.519 934833==    at 0x483B7F3: malloc (in
/usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
==00:00:00:12.519 934833==    by 0x6A94CE: AllocSetAlloc (aset.c:739)
==00:00:00:12.519 934833==    by 0x6B2AA7: palloc (mcxt.c:963)
==00:00:00:12.519 934833==    by 0x486B838: get_raw_page_internal
(rawpage.c:154)
==00:00:00:12.519 934833==    by 0x486BC35: get_raw_page (rawpage.c:62)
==00:00:00:12.519 934833==    by 0x3BACBE: ExecInterpExpr
(execExprInterp.c:699)
==00:00:00:12.519 934833==    by 0x3B7A64: ExecInterpExprStillValid
(execExprInterp.c:1802)
==00:00:00:12.519 934833==    by 0x3C8C3B: ExecEvalExpr (executor.h:294)
==00:00:00:12.519 934833==    by 0x3C8C3B: ExecEvalFuncArgs
(execSRF.c:836)
==00:00:00:12.519 934833==    by 0x3C95C8: ExecMakeTableFunctionResult
(execSRF.c:181)
==00:00:00:12.519 934833==    by 0x3DB7D4: FunctionNext
(nodeFunctionscan.c:95)
==00:00:00:12.519 934833==    by 0x3CA059: ExecScanFetch (execScan.c:133)
==00:00:00:12.519 934833==    by 0x3CA0F4: ExecScan (execScan.c:182)
==00:00:00:12.519 934833== 
{
   <insert_a_suppression_name_here>
   Memcheck:Addr2
   fun:verify_brin_page
   fun:brin_revmap_data
   fun:ExecMakeTableFunctionResult
   fun:FunctionNext
   fun:ExecScanFetch
   fun:ExecScan
   fun:ExecFunctionScan
   fun:ExecProcNodeFirst
   fun:ExecProcNode
   fun:ExecutePlan
   fun:standard_ExecutorRun
   fun:ExecutorRun
   fun:PortalRunSelect
}
2020-07-04 17:57:55.915 MSK [934833] ERROR:  page is not a BRIN page of type
"revmap"
2020-07-04 17:57:55.915 MSK [934833] DETAIL:  Expected special type
0000f092, got 00007f7f.
2020-07-04 17:57:55.915 MSK [934833] STATEMENT:  SELECT * FROM
brin_revmap_data(get_raw_page('test1', 0));

Reproduced on REL_10_STABLE..master.