In such a duplication the previous gets overwritten and leaks

Fixes: memleak
Fixes: 12510/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_PGSSUB_fuzzer-5694439226343424

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 e35c3d88)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>