Fix printing backtrace during generator closing

Fixes GH-15851

Author: iluuu1994

Zend/tests/gh15851.phpt

@@ -0,0 +1,28 @@
+--TEST--
+GH-15851: Access on NULL when printing backtrace with freed generator
+--FILE--
+<?php
+
+class Foo {
+    public function __destruct() {
+        debug_print_backtrace();
+    }
+}
+
+function bar() {
+    yield from foo();
+}
+
+function foo() {
+    $foo = new Foo();
+    yield;
+}
+
+$gen = bar();
+foreach ($gen as $dummy);
+
+?>
+--EXPECTF--
+#0 %s(%d): Foo->__destruct()
+#1 %s(%d): foo()
+#2 %s(%d): bar()

Zend/zend_vm_def.h

@@ -4548,11 +4548,13 @@ ZEND_VM_HANDLER(161, ZEND_GENERATOR_RETURN, CONST|TMP|VAR|CV, ANY, SPEC(OBSERVER
 
 	ZEND_OBSERVER_FCALL_END(generator->execute_data, &generator->retval);
 
-	EG(current_execute_data) = EX(prev_execute_data);
+	zend_execute_data *prev_execute_data = EX(prev_execute_data);
 
 	/* Close the generator to free up resources */
 	zend_generator_close(generator, 1);
 
+	EG(current_execute_data) = prev_execute_data;
+
 	/* Pass execution back to handling code */
 	ZEND_VM_RETURN();
 }