Fix GH-20614: SplFixedArray incorrectly handles references in deserialization

All other code caters to dereferencing array elements, except the
unserialize handler. This causes references to be present in the fixed
array even though this seems not intentional as reference assign is
otherwise impossible.
On 8.5+ this causes an assertion failure. On 8.3+ this causes references
to be present where they shouldn't be.

Closes GH-20616.

NEWS

@@ -61,6 +61,10 @@ PHP                                                                        NEWS
   . Fixed ZPP type violation in phpdbg_get_executable() and phpdbg_end_oplog().
     (Girgias)
 
+- SPL:
+  . Fixed bug GH-20614 (SplFixedArray incorrectly handles references
+    in deserialization). (ndossche)
+
 - Standard:
   . Fix memory leak in array_diff() with custom type checks. (ndossche)
   . Fixed bug GH-20583 (Stack overflow in http_build_query

ext/spl/spl_fixedarray.c

@@ -652,7 +652,7 @@ PHP_METHOD(SplFixedArray, __unserialize)
 		intern->array.size = 0;
 		ZEND_HASH_FOREACH_STR_KEY_VAL(data, key, elem) {
 			if (key == NULL) {
-				ZVAL_COPY(&intern->array.elements[intern->array.size], elem);
+				ZVAL_COPY_DEREF(&intern->array.elements[intern->array.size], elem);
 				intern->array.size++;
 			} else {
 				Z_TRY_ADDREF_P(elem);
@@ -833,7 +833,7 @@ PHP_METHOD(SplFixedArray, offsetGet)
 	value = spl_fixedarray_object_read_dimension_helper(intern, zindex);
 
 	if (value) {
-		RETURN_COPY_DEREF(value);
+		RETURN_COPY(value);
 	} else {
 		RETURN_NULL();
 	}

ext/spl/tests/gh20614.phpt

@@ -0,0 +1,23 @@
+--TEST--
+GH-20614 (SplFixedArray incorrectly handles references in deserialization)
+--FILE--
+<?php
+
+$fa = new SplFixedArray(0);
+$nr = 1;
+$array = [&$nr];
+$fa->__unserialize($array);
+var_dump($fa);
+unset($fa[0]);
+var_dump($fa);
+
+?>
+--EXPECT--
+object(SplFixedArray)#1 (1) {
+  [0]=>
+  int(1)
+}
+object(SplFixedArray)#1 (1) {
+  [0]=>
+  NULL
+}