BUG #15395: Assert failure when using CURRENT OF with inheritance

The following bug has been logged on the website:

Bug reference:      15395
Logged by:          Matvey Arye
Email address:      mat@timescale.com
PostgreSQL version: 10.4
Operating system:   OSX
Description:

The following SQL script causes an Assert failure if PostgreSQL is compiled
with asserts. When run on a PostgreSQL instance with Asserts turned off,
this script seems to work as expected. Thus, I suspect that it may be safe
to just remove the Assert at execCurrent.c:239 but am not familiar enough
with this part of the code to be sure.

The SQL script is:
```
CREATE TABLE current_check (currentid int, payload text);

CREATE TABLE current_check_1 () INHERITS (current_check);
CREATE TABLE current_check_2 () INHERITS (current_check);

INSERT INTO current_check_1 VALUES
    (1, 'abc');
INSERT INTO current_check_2 VALUES
    (2, 'bcd'),
    (3, 'cde'),
    (4, 'def');

BEGIN;

DECLARE current_check_cursor SCROLL CURSOR FOR SELECT * FROM
current_check;

FETCH ABSOLUTE 2 FROM current_check_cursor;
FETCH ABSOLUTE 1 FROM current_check_cursor;

DELETE FROM current_check WHERE CURRENT OF current_check_cursor RETURNING
*;

COMMIT;
```

The backtrace for the Assert is:
```
  * frame #0: 0x00007fff54a19b66 libsystem_kernel.dylib`__pthread_kill +
10
    frame #1: 0x00007fff54be4080 libsystem_pthread.dylib`pthread_kill +
333
    frame #2: 0x00007fff549751ae libsystem_c.dylib`abort + 127
    frame #3: 0x0000000102581630
postgres`ExceptionalCondition(conditionName="!(((bool) (((const
void*)(current_tid) != ((void*)0)) && ((current_tid)->ip_posid !=0))))",
errorType="FailedAssertion", fileName="execCurrent.c", lineNumber=239) at
assert.c:54
    frame #4: 0x00000001021b025f
postgres`execCurrentOf(cexpr=0x00007f890604bf00,
econtext=0x00007f890609c640, table_oid=16402,
current_tid=0x00007ffeedcf4588) atexecCurrent.c:239
    frame #5: 0x00000001021ffbd5
postgres`TidListEval(tidstate=0x00007f890609a6b8) at nodeTidscan.c:224
    frame #6: 0x00000001021ff4de postgres`TidNext(node=0x00007f890609a6b8)
at nodeTidscan.c:329
    frame #7: 0x00000001021cabab
postgres`ExecScanFetch(node=0x00007f890609a6b8, accessMtd=(postgres`TidNext
at nodeTidscan.c:304), recheckMtd=(postgres`TidRecheck at
nodeTidscan.c:420)) at execScan.c:97
    frame #8: 0x00000001021ca8d3 postgres`ExecScan(node=0x00007f890609a6b8,
accessMtd=(postgres`TidNext at nodeTidscan.c:304),
recheckMtd=(postgres`TidRecheck at nodeTidscan.c:420)) at execScan.c:164
    frame #9: 0x00000001021ff082
postgres`ExecTidScan(pstate=0x00007f890609a6b8) at nodeTidscan.c:453
    frame #10: 0x00000001021c8d72
postgres`ExecProcNodeFirst(node=0x00007f890609a6b8) at execProcnode.c:430
    frame #11: 0x00000001021f27f2
postgres`ExecProcNode(node=0x00007f890609a6b8) at executor.h:250
    frame #12: 0x00000001021f182f
postgres`ExecModifyTable(pstate=0x00007f8906098f00) at
nodeModifyTable.c:1645
    frame #13: 0x00000001021c5a82
postgres`ExecProcNode(node=0x00007f8906098f00) at executor.h:250
    frame #14: 0x00000001021c1813
postgres`ExecutePlan(estate=0x00007f8906098840,
planstate=0x00007f8906098f00, use_parallel_mode='\0', operation=CMD_DELETE,
sendTuples='\x01', numberTuples=0, direction=ForwardScanDirection,
dest=0x00007f8906000278, execute_once='\x01') at execMain.c:1722
    frame #15: 0x00000001021c16db
postgres`standard_ExecutorRun(queryDesc=0x00007f8906000310,
direction=ForwardScanDirection, count=0, execute_once='\x01') at
execMain.c:363
    frame #16: 0x00000001021c1497
postgres`ExecutorRun(queryDesc=0x00007f8906000310,
direction=ForwardScanDirection, count=0, execute_once='\x01') at
execMain.c:306
    frame #17: 0x00000001023cfc4e
postgres`ProcessQuery(plan=0x00007f890608eea0, sourceText="DELETE FROM
current_check WHERE CURRENT OF current_check_cursor RETURNING *;",
params=0x0000000000000000, queryEnv=0x0000000000000000,
dest=0x00007f8906000278, completionTag="") at pquery.c:161
    frame #18: 0x00000001023cecb8
postgres`PortalRunMulti(portal=0x00007f8903806040, isTopLevel='\x01',
setHoldSnapshot='\x01', dest=0x00007f8906000278, altdest=0x00000001027d3348,
completionTag="") at pquery.c:1286
    frame #19: 0x00000001023ce6b4
postgres`FillPortalStore(portal=0x00007f8903806040, isTopLevel='\x01') at
pquery.c:1033
    frame #20: 0x00000001023ce285
postgres`PortalRun(portal=0x00007f8903806040, count=9223372036854775807,
isTopLevel='\x01', run_once='\x01', dest=0x00007f890608ef98,
altdest=0x00007f890608ef98, completionTag="") at pquery.c:768
    frame #21: 0x00000001023c9c2c
postgres`exec_simple_query(query_string="DELETE FROM current_check WHERE
CURRENT OF current_check_cursor RETURNING *;") at postgres.c:1099
    frame #22: 0x00000001023c8e37 postgres`PostgresMain(argc=1,
argv=0x00007f8905006d10, dbname="single", username="default_perm_user") at
postgres.c:4088
    frame #23: 0x000000010230f08e
postgres`BackendRun(port=0x00007f89049006c0) at postmaster.c:4405
    frame #24: 0x000000010230e403
postgres`BackendStartup(port=0x00007f89049006c0) at postmaster.c:4077
    frame #25: 0x000000010230d3e5 postgres`ServerLoop at postmaster.c:1755
    frame #26: 0x000000010230ab25 postgres`PostmasterMain(argc=8,
argv=0x00007f8902c03160) at postmaster.c:1363
    frame #27: 0x000000010222e529 postgres`main(argc=8,
argv=0x00007f8902c03160) at main.c:228
```