Fix #81429: Handle resizing in SplFixedArray::offsetSet

offsetSet did not account for the fact that the array may no longer exist after
the field is overwritten. This fixes that.

Add test of resizing both to the empty array and a smaller array - there should
be no valgrind warnings with a proper fix.

Alternate approach to php#7486 (described in https://bugs.php.net/bug.php?id=81429)

Author: TysonAndre

ext/spl/spl_fixedarray.c

@@ -389,10 +389,14 @@ static inline void spl_fixedarray_object_write_dimension_helper(spl_fixedarray_o
 		zend_throw_exception(spl_ce_RuntimeException, "Index invalid or out of range", 0);
 		return;
 	} else {
-		if (!Z_ISUNDEF(intern->array.elements[index])) {
-			zval_ptr_dtor(&(intern->array.elements[index]));
+		/* Fix #81429 */
+		zval *const ptr = &(intern->array.elements[index]);
+		zval tmp;
+		ZVAL_COPY_VALUE(&tmp, ptr);
+		ZVAL_COPY_DEREF(ptr, value);
+		if (!Z_ISUNDEF(tmp)) {
+			zval_ptr_dtor(&tmp);
 		}
-		ZVAL_COPY_DEREF(&intern->array.elements[index], value);
 	}
 }
 /* }}} */

ext/spl/tests/SplFixedArray_setSize_destruct.phpt

@@ -0,0 +1,55 @@
+--TEST--
+SplFixedArray::setSize in offsetSet destructor (#81429)
+--FILE--
+<?php
+$values = new SplFixedArray(1);
+$values->offsetSet(0, new HasDestructor());
+$values->offsetSet(0, false);
+echo "Done\n";
+
+class HasDestructor {
+    public function __destruct() {
+        global $values;
+        var_dump($values);
+        $values->setSize($values->getSize() - 1);
+        var_dump($values);
+    }
+}
+
+$values->setSize(5);
+$values->offsetSet(4, new HasDestructor());
+echo "Done\n";
+--EXPECT--
+object(SplFixedArray)#1 (1) {
+  [0]=>
+  bool(false)
+}
+object(SplFixedArray)#1 (1) {
+  [0]=>
+  bool(false)
+}
+Done
+Done
+object(SplFixedArray)#1 (5) {
+  [0]=>
+  NULL
+  [1]=>
+  NULL
+  [2]=>
+  NULL
+  [3]=>
+  NULL
+  [4]=>
+  object(HasDestructor)#2 (0) {
+  }
+}
+object(SplFixedArray)#1 (4) {
+  [0]=>
+  NULL
+  [1]=>
+  NULL
+  [2]=>
+  NULL
+  [3]=>
+  NULL
+}