Fix crash in Grooveshark.

Not sure how I end up with this null pointer, but better to always to this sanity check.

src/internet/grooveshark/groovesharkservice.cpp

@@ -297,6 +297,7 @@ void GroovesharkService::SearchSongsFinished(QNetworkReply* reply) {
   task_search_id_ = 0;
 
   // Fill results list
+  if (!search_) return;
   for (const Song& song : songs) {
     QStandardItem* child = CreateSongItem(song);
     search_->appendRow(child);