avformat/hlsenc: Add deinit function

avformat/hlsenc: Add deinit function

This fixes memleaks in instances such as:
a) When an allocation fails at one of the two places in hls_init() where
the error is returned immediately without goto fail first.
b) When an error happens when writing the header.
c) When an allocation fails at one of the three places in
hls_write_trailer() where the error is returned immediately without goto
fail first.
d) When one decides not to write the trailer at all (e.g. because of
errors when writing packets).
Furthermore, it removes code duplication and allows to return
immediately, without goto fail first.

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