avformat/aacdec: Fix leak in adts_aac_read_packet()

Fixes: chromium-773637/clusterfuzz-testcase-minimized-6418078673141760

Found-by: ossfuzz/chromium
Reviewed-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
