Verify long_ref_count!=0. Fix issue 281.

Originally committed as revision 11171 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/h264.c

@@ -3662,7 +3662,7 @@ static int decode_ref_pic_marking(H264Context *h, GetBitContext *gb){
         }else{
             assert(h->long_ref_count + h->short_ref_count <= h->sps.ref_frame_count);
 
-            if(h->long_ref_count + h->short_ref_count == h->sps.ref_frame_count &&
+            if(h->short_ref_count && h->long_ref_count + h->short_ref_count == h->sps.ref_frame_count &&
                     !(FIELD_PICTURE && !s->first_field && s->current_picture_ptr->reference)) {
                 h->mmco[0].opcode= MMCO_SHORT2UNUSED;
                 h->mmco[0].short_pic_num= h->short_ref[ h->short_ref_count - 1 ]->frame_num;