Fix GH-15496: Fix accessing NULL pointer on output handler stack

Author: alexandre-daubois

ext/standard/tests/gh15496_original_reproducer.phpt

@@ -49,6 +49,8 @@ while (1) {
 }
 ?>
 --EXPECTF--
-Fatal error: {closure%A}(): Cannot use output buffering in output buffering display handlers in %s on line %d
+Notice: ob_start(): Failed to create buffer in %s on line %d
+
+Fatal error: Allowed memory size of %d bytes exhausted at %s (tried to allocate %d bytes) in %s on line %d
 
 Notice: ob_start(): Failed to create buffer in %s on line %d

main/output.c

@@ -538,7 +538,8 @@ PHPAPI int php_output_handler_start(php_output_handler *handler)
 	if (php_output_lock_error(PHP_OUTPUT_HANDLER_START) || !handler) {
 		return FAILURE;
 	}
-	if (!(OG(flags) & PHP_OUTPUT_ACTIVATED) || !OG(handlers).elements) {
+	/* Output activated but stack destroyed */
+	if ((OG(flags) & PHP_OUTPUT_ACTIVATED) && OG(handlers).elements == NULL) {
 		return FAILURE;
 	}
 	if (NULL != (conflict = zend_hash_find_ptr(&php_output_handler_conflicts, handler->name))) {