avformat/hlsenc: Fix leak of options when initializing muxing fails

avformat/hlsenc: Fix leak of options when initializing muxing fails

hls_mux_init() currently leaks an AVDictionary if opening a dynamic
buffer fails or if avformat_init_output fails. This has been fixed by
moving the initialization resp. the freeing of the dictionary around:
In the former case to a place after opening the dynamic buffer, in the
latter to a place before the check for initialization failure so that it
is done unconditionally.

Furthermore, the dictionary is now only copied and freed if the options
in it are actually used (namely when in SEGMENT_TYPE_FMP4 mode).

Finally, a similar situation in hls_start() has been fixed, too.

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