avformat/hlsenc: Unconditionally free some strings

avformat/hlsenc: Unconditionally free some strings

hls_init() would at first allocate the vtt_basename string, then
allocate the vtt_m3u8_name string followed by several operations that
may fail and then open the subtitles' output context. Yet upon freeing,
these strings were only freed when the subtitles' output context
existed, ensuring that they leak if something goes wrong between their
allocation and the opening of the subtitles' output context. So drop the
check for whether this output context exists.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>