avcodec/fmvc: avoid copying uninitialized data

Fixes: Timeout
Fixes: 30049/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_FMVC_fuzzer-5986909455253504

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

libavcodec/fmvc.c

@@ -440,6 +440,8 @@ static int decode_frame(AVCodecContext *avctx, void *data,
             memcpy(dst, src, avctx->width * s->bpp);
             dst -= frame->linesize[0];
             src += s->stride * 4;
+            if (bytestream2_tell_p(pb) < y*s->stride * 4)
+                break;
         }
     } else {
         unsigned block, nb_blocks;