avcodec/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 behavior of keeping the
packet unchanged in case of error.

Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: James Almer <jamrial@gmail.com>
