src/sulogin.c: Invert logic to reduce indentation

src/sulogin.c: Invert logic to reduce indentation

Also, it was checking for >=0 for success, but since that code is for
opening a different tty as stdin, that was bogus.  But since it's
guaranteed to be either 0 or -1, this commit doesn't add any code to
make sure it's 0 (i.e., we could say !=0 instead of ==-1).  That's more
appropriate for a different commit.

Signed-off-by: Alejandro Colomar <alx@kernel.org>