transcode: don't fail spu track if we don't have something to render

SPU track works even if it doesn't have something to render at given time.
Fixes soverlay and subtitles in sout.

modules/stream_out/transcode/spu.c

@@ -129,7 +129,10 @@ int transcode_spu_process( sout_stream_t *p_stream,
 
     p_subpic = id->p_decoder->pf_decode_sub( id->p_decoder, &in );
     if( !p_subpic )
-        return VLC_EGENERIC;
+    {
+        /* We just don't have anything to handle now, go own*/
+        return VLC_SUCCESS;
+    }
 
     if( p_sys->b_master_sync && p_sys->i_master_drift )
     {