Fix crash when retrieving query hint

This issue would be able to crash the server when for example setting
pg_hint_plan.parse_messages to 'error' in a session and trigger an error
via a hint that cannot be parsed.

Per issue #162.  This issue does not exist in 14 and newer branches
because get_current_hint_string() is much more careful with the handling
of pfree()'d current_hint_str there.

Reported-by: Pogosyan Vardan
Author: Sergei Glukhov
Backpatch-through: 12

pg_hint_plan.c

@@ -3015,7 +3015,10 @@ get_current_hint_string(ParseState *pstate, Query *query)
 		 * use..
 		 */
 		if (current_hint_str)
+		{
 			pfree((void *)current_hint_str);
+			current_hint_str = NULL;
+		}
 
 		oldcontext = MemoryContextSwitchTo(TopMemoryContext);
 		current_hint_str = get_hints_from_comment(query_str);