pgsql: Fix masking of SP-GiST pages during xlog consistency check

Fix masking of SP-GiST pages during xlog consistency check

spg_mask() didn't take into account that pd_lower equal to SizeOfPageHeaderData
is still valid value.  This commit fixes that.  Backpatch to 11, where
spg_mask() pg_lower check was introduced.

Reported-by: Michael Paquier
Discussion: https://postgr.es/m/20200615131405.GM52676%40paquier.xyz
Backpatch-through: 11

Branch
------
REL_13_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/39aafc88c4b4ac281df8b2c2b8be72d4e4d99e9f

Modified Files
--------------
src/backend/access/spgist/spgxlog.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)