Fix re-execution of a failed SQLFunctionCache entry.

Fix re-execution of a failed SQLFunctionCache entry.

If we error out during execution of a SQL-language function, we will
often leave behind non-null pointers in its SQLFunctionCache's cplan
and eslist fields.  This is problematic if the SQLFunctionCache is
re-used, because those pointers will point at resources that were
released during error cleanup.  This problem escaped detection so far
because ordinarily we won't re-use an FmgrInfo+SQLFunctionCache struct
after a query error.  However, in the rather improbable case that
someone implements an opclass support function in SQL language, there
will be long-lived FmgrInfos for it in the relcache, and then the
problem is reachable after the function throws an error.

To fix, add a flag to SQLFunctionCache that tracks whether execution
escapes out of fmgr_sql, and clear out the relevant fields during
init_sql_fcache if so.  (This is going to need more thought if we ever
try to share FMgrInfos across threads; but it's very far from being
the only problem such a project will encounter, since many functions
regard fn_extra as being query-local state.)

This broke at commit 0313c5dc6; before that we did not try to re-use
SQLFunctionCache state across calls.  Hence, back-patch to v18.

Bug: #19026
Reported-by: Alexander Lakhin <exclusion@gmail.com>
Author: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/19026-90aed5e71d0c8af3@postgresql.org
Backpatch-through: 18