When the mov/mp4 demuxer encounters an error during decrypting a packet,
it returns the error, yet doesn't free the packet, so that the packet
leaks. This has been fixed in this commit.

Fixes the memleaks from ticket #8150.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>