avformat/mxfdec: free duplicated utf16 strings

Fixes: memleak
Fixes: 23415/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-5124814510751744

Suggested-by: Marton Balint <cus@passwd.hu>
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

@@ -867,6 +867,7 @@ static inline int mxf_read_utf16_string(AVIOContext *pb, int size, char** str, i
         return AVERROR(EINVAL);
 
     buf_size = size + size / 2 + 1;
+    av_free(*str);
     *str = av_malloc(buf_size);
     if (!*str)
         return AVERROR(ENOMEM);