avformat/hlsenc: Fix memleaks with repeating parameters

avformat/hlsenc: Fix memleaks with repeating parameters

When a parameter like e.g. language is contained more than once in the
part of var_stream_map pertaining to a single VariantStream, the later
one just overwrites the pointer to the earlier one, leading to a
memleak. This commit changes this by handling the situation gracefully:
The earlier string is silently freed first, so that the last one wins.

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