avcodec/h263dec: Sanity check against minimal I/P frame size

Fixes: Timeout
Fixes: 49718/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MPEG4_fuzzer-4874987894341632

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

libavcodec/h263dec.c

@@ -546,6 +546,8 @@ retry:
     avctx->has_b_frames = !s->low_delay;
 
     if (CONFIG_MPEG4_DECODER && avctx->codec_id == AV_CODEC_ID_MPEG4) {
+        if (s->pict_type != AV_PICTURE_TYPE_B && s->mb_num/2 > get_bits_left(&s->gb))
+            return AVERROR_INVALIDDATA;
         if (ff_mpeg4_workaround_bugs(avctx) == 1)
             goto retry;
         if (s->studio_profile != (s->idsp.idct == NULL))