support/mkusers: fix warnings when adding a new user

When adding a new user (or a new group), we would get warnings, like:
  [...]/support/scripts/mkusers: line 145: [: too many arguments

This is because we're checking if a UID (or a GID) is already defined,
and/or is different from the requested one, both checks in the same
test.

Of course, if a UID (or a GID) is not defined, it does not have a value,
so we can not compare it to an integer.

Fix that by splitting the test in two, so the second is only executed if
the first is sucessful.

Reported-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>