avformat/mxfdec: Check avio_read() success in mxf_decrypt_triplet()

Fixes: Use of uninitialized memory
Fixes: 71444/clusterfuzz-testcase-minimized-ffmpeg_dem_MXF_fuzzer-5448597561212928

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

libavformat/mxfdec.c

@@ -697,7 +697,8 @@ static int mxf_decrypt_triplet(AVFormatContext *s, AVPacket *pkt, KLVPacket *klv
     if (size < 32 || size - 32 < orig_size || (int)orig_size != orig_size)
         return AVERROR_INVALIDDATA;
     avio_read(pb, ivec, 16);
-    avio_read(pb, tmpbuf, 16);
+    if (avio_read(pb, tmpbuf, 16) != 16)
+        return AVERROR_INVALIDDATA;
     if (mxf->aesc)
         av_aes_crypt(mxf->aesc, tmpbuf, tmpbuf, 1, ivec, 1);
     if (memcmp(tmpbuf, checkv, 16))