Fix access on NULL when printing backtrace with freed generator

Fixes GH-15851

Author: iluuu1994

Zend/tests/gh15851.phpt

@@ -0,0 +1,38 @@
+--TEST--
+GH-15851: Access on NULL when printing backtrace with freed generator
+--FILE--
+<?php
+
+class Foo {
+    public $obj;
+    public $printBacktrace;
+
+    public function __destruct() {
+        if ($this->printBacktrace) {
+            debug_print_backtrace();
+        }
+    }
+}
+
+function bar() {
+    yield from foo();
+}
+
+function foo() {
+    // Default GC threshold
+    for ($i = 0; $i < 10001; $i++) {
+        $obj = new Foo();
+        $obj->obj = $obj;
+        $obj->printBacktrace = $i === 0;
+    }
+
+    yield;
+}
+
+$gen = bar();
+foreach ($gen as $dummy);
+
+?>
+--EXPECTF--
+#0 %s(%d): Foo->__destruct()
+#1 %s(%d): unknown()

Zend/zend_builtin_functions.c

@@ -1788,7 +1788,7 @@ ZEND_API void zend_fetch_debug_backtrace(zval *return_value, int skip_last, int
 		}
 
 		func = call->func;
-		if (!fake_frame && func->common.function_name) {
+		if (!fake_frame && func && func->common.function_name) {
 			ZVAL_STR_COPY(&tmp, func->common.function_name);
 			_zend_hash_append_ex(stack_frame, ZSTR_KNOWN(ZEND_STR_FUNCTION), &tmp, 1);