fix sound buffer resource leak

Openal doesn't remove buffers unless they were disconnected form
their source.

simgear/sound/sample_group.cxx

@@ -113,6 +113,8 @@ void SGSampleGroup::update( double dt ) {
             sample->stop();
             if ( !sample->is_queue() ) {
                 ALuint buffer = sample->get_buffer();
+                // disconnect buffer from its source - otherwise it cannot be deleted
+                alSourceUnqueueBuffers(sample->get_source(), 1, &buffer);
                 alDeleteBuffers( 1, &buffer );
                 testForALError("buffer remove");
             }

simgear/sound/sample_group.hxx

@@ -180,7 +180,7 @@ public:
     /**
      * Request to stop playing the refered audio sample.
      * @param refname Reference name of the audio sample to stop
-     * @return true if the audio sample exsists and is scheduled to stop
+     * @return true if the audio sample exists and is scheduled to stop
      */
     bool stop( const string& refname );