If ff_subtitles_queue_insert() were given a NULL buffer
with 0 length, it would still attempt to grow the packet
or memcpy depending on if merge option is enabled.

In this commit, allow passing a NULL buffer with 0 length
without attempting to do such operations. This way, if a
subtitle demuxer happens to pass an empty cue or wants to
use av_get_packet() to read bytes, there are no unnecessary
operations on the packet after it is allocated.
Signed-off-by: Marth64 <marth64@proxyid.net>