Fixes: memleak
Fixes: 26450/clusterfuzz-testcase-minimized-ffmpeg_dem_FFMETADATA_fuzzer-6249850443923456

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