avformat/hlsenc: Fix leak of options when writing packets

avformat/hlsenc: Fix leak of options when writing packets

Under certain circumstances hls_write_packet() would add options to an
AVDictionary. Said dictionary was never explicitly freed, instead it was
presumed that these options would be consumed when opening a new
IO-context. This left several possibilities for memleaks:

a) When no new IO-context would be opened at all. This is possible when
using both the flags temp_file and single_file together with a file
output.
b) When an error happens before one actually tries to open the new
IO-context.
c) When the new IO-context does not consume all options.

All three have been fixed; furthermore, the AVDictionary has been put
into a smaller scope (namely the only part of hls_write_packet() where
it is actually used).

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