Fix improper error checking

Currently all (HTML/TEXT) spell checking logs an error (due commit b15cc7d) and HTML spell checking doesn't work (due commit cfe2fc8). Fix: ignore error="0" value from the returned XML as it isn't an error.

program/lib/Roundcube/rcube_spellchecker.php

@@ -377,7 +377,7 @@ class rcube_spellchecker
         if (!$store) {
             $this->error = "Empty result from spelling engine";
         }
-        else if (preg_match('/<spellresult error="([^"]+)"/', $store, $m)) {
+        else if (preg_match('/<spellresult error="((?!0")[^"]+)"/', $store, $m)) {
             $this->error = "Error code $m[1] returned";
         }