MythMusic: fix Coverity ID 1026166 Improper use of negative value

In Ripper::toggleTrackActive(MythUIButtonListItem *): Negative value can be
returned from function is not being checked before being used improperly

The only way this could be a problem is if item doesn't exist which is very
unlikely.

mythplugins/mythmusic/mythmusic/cdrip.cpp

@@ -1294,7 +1294,13 @@ void Ripper::toggleTrackActive(MythUIButtonListItem *item)
     if (m_tracks->isEmpty() || !item)
         return;
 
-    RipTrack *track = m_tracks->at(m_trackList->GetItemPos(item));
+    int pos = m_trackList->GetItemPos(item);
+
+    // sanity check item position
+    if (pos < 0 || pos > m_tracks->count() - 1)
+        return;
+
+    RipTrack *track = m_tracks->at(pos);
 
     if (!track->active && !track->isNew)
     {