Fix double-free in pg_stat_autovacuum_scores.

Fix double-free in pg_stat_autovacuum_scores.

Presently, relation_needs_vacanalyze() unconditionally frees the
pgstat entry returned by pgstat_fetch_stat_tabentry_ext().  This
behavior was first added by commit 02502c1bca to avoid memory
leakage in autovacuum.  While this is fine for autovacuum since it
forces stats_fetch_consistency to "none", it is not okay for other
callers that use "cache" or "snapshot".  This manifests as a
double-free when pg_stat_autovacuum_scores is called multiple times
in the same transaction.

To fix, add a "bool *may_free" parameter to
pgstat_fetch_stat_tabentry_ext() that returns whether it is safe
for the caller to explicitly pfree() the result.  If a caller would
rather leave it to the memory context machinery to free the result,
it can pass NULL as the "may_free" argument (or just ignore its
value).

Oversight in commit 87f61f0c82.

Reported-by: Tender Wang <tndrwang@gmail.com>
Reported-by: Alexander Lakhin <exclusion@gmail.com>
Suggested-by: Andres Freund <andres@anarazel.de>
Suggested-by: Tom Lane <tgl@sss.pgh.pa.us>
Author: Sami Imseih <samimseih@gmail.com>
Discussion: https://postgr.es/m/CAHewXNkJKdwb3D5OnksrdOqzqUnXUEMpDam1TPW0vfUkW%3D7jUw%40mail.gmail.com
Discussion: https://postgr.es/m/5684f479-858e-4c5d-b8f5-bcf05de1f909%40gmail.com