avformat/mxfdec: cleanup if parsing the header metadata fails

Fixes memleaks
Fixes: asan_static-oob_87d116_10_201.mxf
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/mxfdec.c

@@ -2294,7 +2294,8 @@ static int mxf_read_header(AVFormatContext *s)
                 }
                 if (res < 0) {
                     av_log(s, AV_LOG_ERROR, "error reading header metadata\n");
-                    return res;
+                    ret = res;
+                    goto fail;
                 }
                 break;
             } else {