set time_base for variable fps too

Originally committed as revision 4187 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/h264.c

@@ -3847,7 +3847,7 @@ static int decode_slice_header(H264Context *h){
         if(!s->avctx->sample_aspect_ratio.den)
             s->avctx->sample_aspect_ratio.den = 1;
 
-        if(h->sps.timing_info_present_flag && h->sps.fixed_frame_rate_flag){
+        if(h->sps.timing_info_present_flag){
             s->avctx->time_base= (AVRational){h->sps.num_units_in_tick, h->sps.time_scale};
         }
     }