Fix parallel query crashes by adding NULL checks for exec_state_call_stack

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

This fix adds NULL checks to:
- plsql_TriggerRecursiveCheck() in hooks.c - returns false if NULL
- find_innermost_catch_block() in err_handler.c - returns NULL if NULL

These functions are now safe to call from parallel workers and will
simply return early without accessing the exec_state_call_stack.

Author: monk0707

contrib/babelfishpg_tsql/src/err_handler.c

@@ -429,8 +429,15 @@ do_error_mapping(PLtsql_estate_err *err)
 static PLtsql_execstate *
 find_innermost_catch_block(void)
 {
-	PLExecStateCallStack *stack = exec_state_call_stack;
-	PLtsql_execstate *estate = stack->estate;
+	PLExecStateCallStack *stack;
+	PLtsql_execstate *estate;
+
+	/* Safety check for parallel workers where exec_state_call_stack may be NULL */
+	if (exec_state_call_stack == NULL)
+		return NULL;
+
+	stack = exec_state_call_stack;
+	estate = stack->estate;
 
 	while ((!estate || !estate->cur_error || !estate->cur_error->error) &&
 		   stack->next)

contrib/babelfishpg_tsql/src/hooks.c

@@ -1687,6 +1687,9 @@ 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)
 	{