In RV3/4 nonzero MB offset for the first slice also indicates error

Originally committed as revision 17201 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/rv34.c

@@ -1402,7 +1402,7 @@ int ff_rv34_decode_frame(AVCodecContext *avctx,
         return -1;
     }
     init_get_bits(&s->gb, buf+get_slice_offset(avctx, slices_hdr, 0), buf_size-get_slice_offset(avctx, slices_hdr, 0));
-    if(r->parse_slice_header(r, &r->s.gb, &si) < 0){
+    if(r->parse_slice_header(r, &r->s.gb, &si) < 0 || si.start){
         av_log(avctx, AV_LOG_ERROR, "First slice header is incorrect\n");
         return -1;
     }