avpacket: fix leak on realloc in av_packet_add_side_data()

If realloc fails, the pointer is overwritten and the previously allocated buffer
is leaked, which goes against the expected functionality of keeping the packet
unchanged in case of error.

Signed-off-by: James Almer <jamrial@gmail.com>
Signed-off-by: Anton Khirnov <anton@khirnov.net>