Don't close the fd if libinput_add_fd() fails

Don't close the fd if libinput_add_fd() fails

Let the caller decide what to do with the fd.

In the current code the caller can't know if the fd was closed on error since
we return NULL on malloc failure as well as on epoll_ctl() failure. In the
latter case the fd was closed, not in the former. The caller had to close
the fd anyway (and all three callers do), so drop closing the fd from this
function.

Found by Coverity.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>