avformat/hlsenc: Check that data is set

If codecpar->extradata is not set (for example, when the stream goes
through the 'tee' muxer), then a segfault occurs.
This patch ensures the data variable is not null before attempting
to access it
Before the var_stream_map option was available - I was using the tee
muxer to create each resolution as an individual stream.
When running this configuration after the most recent hlsenc change
I hit a segfault
The most simple command which recreates the segfault is:
ffmpeg -i in.ts -map 0:a -map 0:v -c:a aac -c:v h264 -f tee [select=\'a,v\':f=hls]tv_hls_hd.m3u8

Signed-off-by: Brendan McGrath <redmcg@redmandi.dyndns.org>
