avformat/segment: Fix leak and invalid free of AVIOContext

avformat/segment: Fix leak and invalid free of AVIOContext

seg_init() and seg_write_header() currently contain a few error paths
in which an already opened AVIOContext for the child muxer leaks (namely
if there are unrecognized options for the child muxer or if writing the
header of the child muxer fails); the reason for this is that this
AVIOContext is not closed in the deinit function. If all goes well, it
is closed when writing the trailer. From this it also follows that the
AVIOContext also leaks when the trailer is never written, even when
writing the header succeeds.

But simply freeing said AVIOContext in the deinit function is
complicated by the fact that the AVIOContext may or may not have been
opened via the io_open callback: If options are set to discard header
and trailer, said AVIOContext can also be a null context which must not
be closed via the io_close callback. This may lead to crashes, as
io_close may presume the AVIOContext's opaque to be set. It currently
works with the default io_close callback which simply calls avio_close(),
because avio_close() doesn't care about opaque being NULL since commit
6e8e8431e15a58aa44cfdd8c11f9ea096837c0fa. Therefore this commit records
which of the two kinds of AVIOContext is currently in use to use the
right way to close it.

Finally there was one instance (namely if initializing the child muxer
fails with no unrecognized options) where the AVIOContext was always
closed via the io_close callback. The above remark applies to this; it
has been fixed, too.

Reviewed-by: Ridley Combs <rcombs@rcombs.me>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>