src/login.c: Fix off-by-one bugss

src/login.c: Fix off-by-one bugss

These functions expect a size, not a length.  Don't subtract 1 to the
size.

Link: <https://github.com/shadow-maint/shadow/pull/935>
Link: <https://github.com/shadow-maint/shadow/issues/920#issuecomment-1926002209>
Link: <https://github.com/shadow-maint/shadow/pull/757>
Link: <https://github.com/shadow-maint/shadow/issues/674>
See-also: 0656a90bfd0d ("src/login.c: Fix off-by-one buggs")
See-also: 403a2e3771be ("lib/chkname.c: Take NUL byte into account")
Fixes: 3b7cc053872c ("lib: replace `USER_NAME_MAX_LENGTH` macro")
Reviewed-by: Iker Pedrosa <ipedrosa@redhat.com>
Cc: Tobias Stoeckmann <tobias@stoeckmann.org>
Cc: Serge Hallyn <serge@hallyn.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>