avcodec/wmv2dec: Skip I frame if its smaller than 1/8 of the minimal size

Frames that small are not valid and of limited use for error concealment, while
being very computationally intensive to process.

Fixes: Timeout
Fixes: 11168/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_WMV2_fuzzer-5733782032744448

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/wmv2dec.c

@@ -181,6 +181,14 @@ int ff_wmv2_decode_secondary_picture_header(MpegEncContext *s)
             }
 
             s->dc_table_index = get_bits1(&s->gb);
+
+            // at minimum one bit per macroblock is required at least in a valid frame,
+            // we discard frames much smaller than this. Frames smaller than 1/8 of the
+            // smallest "black/skip" frame generally contain not much recoverable content
+            // while at the same time they have the highest computational requirements
+            // per byte
+            if (get_bits_left(&s->gb) * 8LL < (s->width+15)/16 * ((s->height+15)/16))
+                return AVERROR_INVALIDDATA;
         }
         s->inter_intra_pred = 0;
         s->no_rounding      = 1;