sound: fix "source" resource leak

Sound resources were eaten up when a sample could not be loaded.
Restores sound for aircraft attempting to use stereo files for sound effects.
However, when loading fails, sound code still tries to load the same file
again and again (always resulting in the same "stereo files not supported"
error), which slows down simulation.

simgear/sound/sample_group.cxx

@@ -156,7 +156,10 @@ void SGSampleGroup::update( double dt ) {
                 else
                 {
                     if (_smgr->request_buffer(sample) == SGSoundMgr::NO_BUFFER)
+                    {
+                        _smgr->release_source(source);
                         continue;
+                    }
 
                     // start playing the sample
                     ALuint buffer = sample->get_buffer();