avformat/mxfdec: Check that key was read sucessfull

Fixes: use of uninitialized value
Fixes: 70932/clusterfuzz-testcase-minimized-ffmpeg_dem_MXF_fuzzer-4870202133643264

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

@@ -1556,7 +1556,8 @@ static int mxf_read_indirect_value(void *arg, AVIOContext *pb, int size)
     if (size <= 17)
         return 0;
 
-    avio_read(pb, key, 17);
+    if (avio_read(pb, key, 17) != 17)
+        return AVERROR_INVALIDDATA;
     /* TODO: handle other types of of indirect values */
     if (memcmp(key, mxf_indirect_value_utf16le, 17) == 0) {
         return mxf_read_utf16le_string(pb, size - 17, &tagged_value->value);