Make some functions also work on error connections

Make some functions also work on error connections

There is no technical reason why xcb_get_setup() and xcb_get_file_descriptor()
shouldn't work on non-static error connections. They cannot be used for many
useful things, but at least they work.

This works around bugs in lots of programs out there which assume that
xcb_get_setup() does not return NULL and which just happily dereference the
results. Since xcb_connect() never returns NULL, it's a bit weird that
xcb_get_setup() can do so. xcb_get_file_descriptor() is just modified since this
can be done here equally easily and because the fd isn't closed until the final
xcb_disconnect() on the error connection.

Non-static error connections are connections which entered an error state after
xcb_connect() succeeded. If something goes wrong in establishing a connection,
xcb_connect() will return a static error connection which doesn't have the
fields used here.

Signed-off-by: Uli Schlachter <psychon@znc.in>