pg_stat_statements: fetch stmt location/length before it disappears.

When executing a utility statement, we must fetch everything
we need out of the PlannedStmt data structure before calling
standard_ProcessUtility.  In certain cases (possibly only ROLLBACK
in extended query protocol), that data structure will get freed
during command execution.  The situation is probably often harmless
in production builds, but in debug builds we intentionally overwrite
the freed memory with garbage, leading to picking up garbage values
of statement location and length, typically causing an assertion
failure later in pg_stat_statements.  In non-debug builds, if
something did go wrong it would likely lead to storing garbage
for the query string.

Report and fix by zhaoqigui (with cosmetic adjustments by me).
It's an old problem, so back-patch to all supported versions.

Discussion: https://postgr.es/m/17663-a344fd0675f92128@postgresql.org
Discussion: https://postgr.es/m/1667307420050.56657@hundsun.com

contrib/pg_stat_statements/pg_stat_statements.c

@@ -1077,6 +1077,8 @@ pgss_ProcessUtility(PlannedStmt *pstmt, const char *queryString,
 {
 	Node	   *parsetree = pstmt->utilityStmt;
 	uint64		saved_queryId = pstmt->queryId;
+	int			saved_stmt_location = pstmt->stmt_location;
+	int			saved_stmt_len = pstmt->stmt_len;
 
 	/*
 	 * Force utility statements to get queryId zero.  We do this even in cases
@@ -1142,6 +1144,16 @@ pgss_ProcessUtility(PlannedStmt *pstmt, const char *queryString,
 		}
 		PG_END_TRY();
 
+		/*
+		 * CAUTION: do not access the *pstmt data structure again below here.
+		 * If it was a ROLLBACK or similar, that data structure may have been
+		 * freed.  We must copy everything we still need into local variables,
+		 * which we did above.
+		 *
+		 * For the same reason, we can't risk restoring pstmt->queryId to its
+		 * former value, which'd otherwise be a good idea.
+		 */
+
 		INSTR_TIME_SET_CURRENT(duration);
 		INSTR_TIME_SUBTRACT(duration, start);
 
@@ -1166,8 +1178,8 @@ pgss_ProcessUtility(PlannedStmt *pstmt, const char *queryString,
 
 		pgss_store(queryString,
 				   saved_queryId,
-				   pstmt->stmt_location,
-				   pstmt->stmt_len,
+				   saved_stmt_location,
+				   saved_stmt_len,
 				   PGSS_EXEC,
 				   INSTR_TIME_GET_MILLISEC(duration),
 				   rows,