avformat/hdsenc: Add explicit deinit function

avformat/hdsenc: Add explicit deinit function

hdsenc already had an explicit function to free all allocations in case
of an error, but it was not marked as deinit function, so that it was
not called automatically when the AVFormatContext for muxing gets freed.

Using an explicit deinit function also makes the code cleaner by
allowing to return immediately without "goto fail".

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