correctly check hash parameter in media dispatcher FS#2648

lib/exe/fetch.php

@@ -157,7 +157,7 @@ function checkFileStatus(&$media, &$file, $rev='') {
   //media to local file
   if(preg_match('#^(https?)://#i',$media)){
     //check hash
-    if(substr(md5(auth_cookiesalt().$media),0,6) != $INPUT->str('hash')){
+    if(substr(md5(auth_cookiesalt().$media),0,6) !== $INPUT->str('hash')){
       return array( 412, 'Precondition Failed');
     }
     //handle external images