Fix access on NULL when printing backtrace with freed generator

[iluuu1994]

Fixes GH-15851

Master additionally needs:

diff --git a/Zend/zend_builtin_functions.c b/Zend/zend_builtin_functions.c
index 9da4a76825..b7595ce7d3 100644
--- a/Zend/zend_builtin_functions.c
+++ b/Zend/zend_builtin_functions.c
@@ -1895,7 +1895,7 @@ ZEND_API void zend_fetch_debug_backtrace(zval *return_value, int skip_last, int
 		}
 
 		/* For frameless calls we add an additional frame for the call itself. */
-		if (ZEND_USER_CODE(call->func->type)) {
+		if (call->func && ZEND_USER_CODE(call->func->type)) {
 			const zend_op *opline = call->opline;
 			if (!ZEND_OP_IS_FRAMELESS_ICALL(opline->opcode)) {
 				goto not_frameless_call;