Revert "IteratorIterator::getInnerIterator() can't return null"

This reverts commit c252420.

This (currently) does not hold for invalid AppendIterators,
revert for now.

Author: nikic

ext/spl/spl_iterators.c

@@ -1447,9 +1447,13 @@ PHP_METHOD(IteratorIterator, getInnerIterator)
 
 	SPL_FETCH_AND_CHECK_DUAL_IT(intern, ZEND_THIS);
 
-	zval *value = &intern->inner.zobject;
-	ZEND_ASSERT(!Z_ISUNDEF_P(value));
-	ZVAL_COPY_DEREF(return_value, value);
+	if (!Z_ISUNDEF(intern->inner.zobject)) {
+		zval *value = &intern->inner.zobject;
+
+		ZVAL_COPY_DEREF(return_value, value);
+	} else {
+		RETURN_NULL();
+	}
 } /* }}} */
 
 static inline void spl_dual_it_free(spl_dual_it_object *intern)

ext/spl/spl_iterators.stub.php

@@ -117,7 +117,7 @@ class IteratorIterator implements OuterIterator
 {
     public function __construct(Traversable $iterator, ?string $class = null) {}
 
-    /** @return Iterator */
+    /** @return Iterator|null */
     public function getInnerIterator() {}
 
     /** @return void */

ext/spl/spl_iterators_arginfo.h

@@ -1,5 +1,5 @@
 /* This is a generated file, edit the .stub.php file instead.
- * Stub hash: d1cb33b7c3d1b480b68c6e52672a8ee2a3b382ef */
+ * Stub hash: de49628718b5fff1a3b2516cc89108e01b67c312 */
 
 ZEND_BEGIN_ARG_INFO_EX(arginfo_class_EmptyIterator_current, 0, 0, 0)
 ZEND_END_ARG_INFO()