useradd: fix write_full() return value

useradd: fix write_full() return value

write_full() returns -1 on error and useradd was checking another value.

Closes: https://github.com/shadow-maint/shadow/issues/1072
Fixes: f45498a6c286 ("libmisc/write_full.c: Improve write_full()")
Reported-by: <https://github.com/brown-midas>
Suggested-by: <https://github.com/brown-midas>
Reviewed-by: Alejandro Colomar <alx@kernel.org>
Reviewed-by: Iker Pedrosa <ipedrosa@redhat.com>