pgsql: Fix crash when reading stats file at startup with fixed-sized ki

Fix crash when reading stats file at startup with fixed-sized kinds

pgstat_read_statsfile() has never checked that the kind of a hash entry
is variable-sized. Since 44539786b829, that has begun tracking which
hash table a kind ID should use, it is possible to finish with a NULL
pointer dereference for a PGSTAT_FILE_ENTRY_HASH. Fixed-sized stats
kinds have no hash table and no DSA area, hence their dsa references in
pgStatLocal.kind_hash[] are NULL.

This case can occur when reading statistics files if the same kind ID is
reused across server restarts, with shared_preload_libraries loading
different stats kind information each time: stats written with a set of
PGSTAT_FILE_ENTRY_HASH, then read back with the same kind ID pointing to
a custom fixed-sized kind info.

Oversight in 44539786b829.

Reported-by: Zsolt Parragi <zsolt(dot)parragi(at)percona(dot)com>
Author: Sami Imseih <samimseih(dot)pg(at)gmail(dot)com>
Author: Zsolt Parragi <zsolt(dot)parragi(at)percona(dot)com>
Discussion: https://postgr.es/m/CAN4CZFMxuOOuYV14+i3YcPG-16q02eBtSVa82J111=ZnDwSicA@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/0d110901f0ef362fcf24b07357c20745f971c602

Modified Files
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src/backend/utils/activity/pgstat.c | 8 ++++++++
1 file changed, 8 insertions(+)