Fix segfault on recording with old FFmpeg

The AVPacket fields side_data and side_data_elems were not initialized
by av_packet_ref() in old FFmpeg versions (prior to [1]).

As a consequence, on av_packet_unref(), side_data was freed, causing a
segfault.

Fixes <https://github.com/Genymobile/scrcpy/issues/707>

[1]: <http://git.videolan.org/gitweb.cgi/ffmpeg.git/?p=ffmpeg.git;a=commitdiff;h=3b4026e15110547892d5d770b6b43c9e34df458f>