pg_stat_statements: Fix potential use-after-free of PlannedStmt

pg_stat_statements: Fix potential use-after-free of PlannedStmt

pgss_ProcessUtility() included a reference to a portion of a PlannedStmt
after the point where this data's structure could have been freed,
causing an incorrect memory access.  There was a comment documenting
this requirement, missed in 3357471cf9f5.

This commit includes a test able to make valgrind complain with a
PlannedStmt freed by an internal ROLLBACK query.  Similarly to what is
mentioned in 495e73c2079e, this can be triggered by using the extended
query protocol, something that can be now tested thanks to the recent
meta-command additions in psql.  This commit mentions potential other
cases, but as far as I can see the extended protocol case with an
internal ROLLBACK is the only problematic pattern reachable in practice.

Issue introduced by 3357471cf9f5, gone unnoticed due to a lack of test
coverage.  The fix is authored by Chao, my contribution being the new
test.

Author: Chao Li <li.evan.chao@gmail.com>
Co-authored-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/2F91906A-F2B5-4A6B-9695-D136957D4545@gmail.com