Simplify and fix generator tree management

Author: nikic

Zend/tests/generators/bug80240.phpt

@@ -0,0 +1,28 @@
+--TEST--
+Bug #80240: Use after free multi yield from
+--FILE--
+<?php
+
+function gen() {
+    yield 0;
+    yield from gen();
+}
+
+function bar($gen) {
+    yield from $gen;
+}
+
+$gen = gen();
+$a = bar($gen);
+$b = bar($gen);
+$a->rewind();
+$b->rewind();
+$a->next();
+unset($gen);
+unset($a);
+unset($b);
+
+?>
+===DONE===
+--EXPECT--
+===DONE===