fix valgrind error

php/ext/google/protobuf/def.c

@@ -504,13 +504,15 @@ static zend_class_entry *Descriptor_GetGeneratedClass(const upb_MessageDef *m) {
   zend_string *str = zend_string_init(classname, strlen(classname), 0);
   zend_class_entry *ce = zend_lookup_class(str);  // May autoload the class.
 
+  zend_string_release (str);
+
   if (!ce) {
     char *classname2 =
       GetPhpClassname(upb_MessageDef_File(m), upb_MessageDef_FullName(m), true);
-    str = zend_string_init(classname2, strlen(classname2), 0);
-    ce = zend_lookup_class(str);  // May autoload the class.
+    zend_string *str2 = zend_string_init(classname2, strlen(classname2), 0);
+    ce = zend_lookup_class(str2);  // May autoload the class.
 
-    zend_string_release (str);
+    zend_string_release (str2);
     free(classname2);
 
     if (!ce) {