openal: fix leak of sound data when uncache_sound on stream

This is an addendum to cd2ea97b1ffb65512f5ee8ba0665f46345ef7795 (fix for #428) which did
not properly delete the SoundData when uncaching sounds that were loaded as streams.

panda/src/audiotraits/openalAudioManager.cxx

@@ -553,6 +553,7 @@ uncache_sound(const Filename &file_name) {
       if (sd->_movie->get_filename() == path ||
           sd->_movie->get_filename() == file_name) {
         exqi = _expiring_streams.erase(exqi);
+        delete sd;
         continue;
       }
     }