Fixes: memleak on error path
Fixes: 15984/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-5679918412726272

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit da503941)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>