avpacket: Fixing side data copy when src == dst

Fixing av_packet_copy_side_data to work correctly when source and
destination are the same. This makes sure that there is no memory
leak and double frees.

Signed-off by: Vignesh Venkatasubramanian <vigneshv@google.com>

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
