spl_ptr_llist_offset() is not supposed to return NULL here

This issue has found by a static analyzer.

Closes GH-8272.

Author: zhou1615

ext/spl/spl_dllist.c

@@ -1191,6 +1191,7 @@ PHP_METHOD(SplDoublyLinkedList, add)
 
 		/* Get the element we want to insert before */
 		element = spl_ptr_llist_offset(intern->llist, index, intern->flags & SPL_DLLIST_IT_LIFO);
+		ZEND_ASSERT(element != NULL);
 
 		ZVAL_COPY(&elem->data, value);
 		SPL_LLIST_RC(elem) = 1;