avformat/hlsenc: Only allocate when data is known to be needed

avformat/hlsenc: Only allocate when data is known to be needed

hls_init() would allocate a buffer, although it is only needed in one of
two branches that follow. This commit moves the allocation to the branch
that actually needs the buffer.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Reviewed-by: Steven Liu <lq@onvideo.cn>