Clear buffers before deleting the sources

This fixes a very long lasting shutdown procedure which made it look
like Quake II freezed. Especially on slow machines it was a problem.

src/client/sound/snd_al.c

@@ -145,6 +145,8 @@ fail:
 void AL_Shutdown( void ) {
     Com_Printf( "Shutting down OpenAL.\n" );
 
+    S_AL_StreamDie();
+
 	qalDeleteSources(1, &streamSource);
 	qalDeleteFilters(1, &underwaterFilter);
 
@@ -154,7 +156,6 @@ void AL_Shutdown( void ) {
         memset( s_srcnums, 0, sizeof( s_srcnums ) );
         s_numchannels = 0;
     }
-    S_AL_StreamDie();
     QAL_Shutdown();
 }