h264: fix the check for invalid SPS:num_ref_frames.

This patch set the limit to 16.

For information, thoses previous commits:
41f7e2d1
5cbb0e70
assumed it was either 30 or 32.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/h264_ps.c

@@ -368,7 +368,7 @@ int ff_h264_decode_seq_parameter_set(H264Context *h){
     }
 
     sps->ref_frame_count= get_ue_golomb_31(&s->gb);
-    if(sps->ref_frame_count > MAX_PICTURE_COUNT-2 || sps->ref_frame_count >= 32U){
+    if(sps->ref_frame_count > MAX_PICTURE_COUNT-2 || sps->ref_frame_count > 16U){
         av_log(h->s.avctx, AV_LOG_ERROR, "too many reference frames\n");
         goto fail;
     }