avformat/mvdec: Check if name was fully read

Fixes: use of uninitialized value
Fixes: 70901/clusterfuzz-testcase-minimized-ffmpeg_dem_MV_fuzzer-6341913949569024

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 4e39795c75)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/mvdec.c

@@ -254,7 +254,8 @@ static int read_table(AVFormatContext *avctx, AVStream *st,
         if (avio_feof(pb))
             return AVERROR_EOF;
 
-        avio_read(pb, name, 16);
+        if (avio_read(pb, name, 16) != 16)
+            return AVERROR_INVALIDDATA;
         name[sizeof(name) - 1] = 0;
         size = avio_rb32(pb);
         if (size < 0) {