Fix parallel query crashes by adding NULL check for exec_state_call_stack in plsql_TriggerRecursiveCheck

In parallel workers, the exec_state_call_stack global variable is NULL
because parallel workers don't have the PL/tsql execution context.
When the plsql_TriggerRecursiveCheck hook is called from parallel
workers during query execution, it would crash trying to dereference
the NULL pointer.

This fix adds a NULL check to plsql_TriggerRecursiveCheck() in hooks.c
that returns false (not a recursive trigger) if exec_state_call_stack
is NULL. This is the safe default behavior.

Author: monk0707

contrib/babelfishpg_tsql/src/hooks.c

@@ -1682,6 +1682,9 @@ plsql_TriggerRecursiveCheck(ResultRelInfo *resultRelInfo)
 		return false;
 	if (pltsql_recursive_triggers)
 		return false;
+	/* Safety check for parallel workers where exec_state_call_stack may be NULL */
+	if (exec_state_call_stack == NULL)
+		return false;
 	cur = exec_state_call_stack;
 	while (cur != NULL)
 	{