avformat/mvdec: Fix DoS due to lack of eof check

Fixes: loop.mv

Found-by: Xiaohei and Wangchu from Alibaba Security Team
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 4f05e2e2dc)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/mvdec.c

@@ -338,6 +338,8 @@ static int mv_read_header(AVFormatContext *avctx)
             uint32_t pos   = avio_rb32(pb);
             uint32_t asize = avio_rb32(pb);
             uint32_t vsize = avio_rb32(pb);
+            if (avio_feof(pb))
+                return AVERROR_INVALIDDATA;
             avio_skip(pb, 8);
             av_add_index_entry(ast, pos, timestamp, asize, 0, AVINDEX_KEYFRAME);
             av_add_index_entry(vst, pos + asize, i, vsize, 0, AVINDEX_KEYFRAME);