Add NULL guards for exec_state_call_stack in pl_exec.c and pl_exec-2.c

Three remaining crash sites after commit 3b06bdd (which covered
iterative_exec.c) still dereference exec_state_call_stack without
a NULL check:

1. pl_exec.c:1283 (pltsql_exec_trigger): pltsql_exec_trigger never
   pushes exec_state_call_stack -- only pltsql_exec_function does at
   line 4475. When a trigger fires from a pure SQL DML context with
   no outer PL/tsql function, exec_state_call_stack is NULL. Add NULL
   check before the trigger_error field dereference.

2. pl_exec.c:10301 (pltsql_estate_cleanup): INSERT EXEC cleanup paths
   can call this function with an empty stack. Add early return when
   exec_state_call_stack is NULL to prevent crash on ->next dereference.

3. pl_exec-2.c:3365 (exec_stmt_usedb): The loop traverses the call
   stack looking for EXEC_BATCH to suppress the database context
   change notification. Guard the initial ->next access with a ternary;
   a NULL stack means no EXEC_BATCH above us, so fall through normally
   to send the notification (do NOT return early here).

Also revert proc_ownership_chaining-vu-verify.out: accepting a
permission denied error for EXEC p6030_8_3c is a regression. The
CTE rewrite SPI call runs under the caller's security context rather
than the procedure owner's, breaking ownership chaining for INSERT
EXEC. Root-cause fix (security context switching in exec_stmt_execsql)
will follow as a separate commit.

Author: monk0707

contrib/babelfishpg_tsql/src/pl_exec-2.c

@@ -3362,7 +3362,7 @@ exec_stmt_usedb(PLtsql_execstate *estate, PLtsql_stmt_usedb *stmt)
 
 	set_cur_user_db_and_path(stmt->db_name, false);
 
-	top_es_entry = exec_state_call_stack->next;
+	top_es_entry = (exec_state_call_stack != NULL) ? exec_state_call_stack->next : NULL;
 	while (top_es_entry != NULL)
 	{
 		/*

contrib/babelfishpg_tsql/src/pl_exec.c

@@ -1280,7 +1280,9 @@ pltsql_exec_trigger(PLtsql_function *func,
 		/*
 		 * If an error was encountered when executing trigger.
 		 */
-		if (support_tsql_trans && !pltsql_disable_txn_in_triggers && exec_state_call_stack->error_data.trigger_error)
+		if (support_tsql_trans && !pltsql_disable_txn_in_triggers &&
+			exec_state_call_stack != NULL &&
+			exec_state_call_stack->error_data.trigger_error)
 			ereport(ERROR,
 					(errcode(ERRCODE_TRIGGERED_ACTION_EXCEPTION),
 					 errmsg("An error was raised during trigger execution. The batch has been aborted and the user transaction, if any, has been rolled back.")));
@@ -10298,6 +10300,9 @@ pltsql_estate_cleanup(void)
 {
 	PLExecStateCallStack *top_es_entry;
 
+	if (exec_state_call_stack == NULL)
+		return;
+
 	top_es_entry = exec_state_call_stack->next;
 	if (top_es_entry != NULL)
 		pltsql_copy_exec_error_data(&(exec_state_call_stack->error_data),

test/JDBC/expected/proc_ownership_chaining-vu-verify.out

@@ -27,10 +27,6 @@ int
 
 EXEC p6030_8_3c;
 GO
-~~ERROR (Code: 33557097)~~
-
-~~ERROR (Message: permission denied for table t6030_22)~~
-
 
 -- CASE 8.4: Function referring table (success: function->table chain)
 SELECT * FROM f6030_8_4();