Add missing volatile qualifier. (#17521)

According to C99 7.13.2.1[^1],

> All accessible objects have values, and all other components of the
abstract machine have state, as of the time the longjmp function was
called, except that the values of objects of automatic storage duration
that are local to the function containing the invocation of the
corresponding setjmp macro that do not have volatile-qualified type and
have been changed between the setjmp invocation and longjmp call are
indeterminate.

The object oldcontext is changed in line 1194 (inside PG_TRY() block)
and read in line 1434 (inside PG_CATCH() block).  We should qualify it
with volatile.

[^1]: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf

src/backend/executor/nodeSubplan.c

@@ -1107,7 +1107,7 @@ ExecSetParamPlan(SubPlanState *node, ExprContext *econtext, QueryDesc *queryDesc
 	SubLinkType subLinkType = subplan->subLinkType;
 	EState	   *estate = planstate->state;
 	ScanDirection dir = estate->es_direction;
-	MemoryContext oldcontext = NULL;
+	volatile MemoryContext oldcontext = NULL;
 	TupleTableSlot *slot;
 	ListCell   *pvar;
 	ListCell   *l;