lib/chkname.c: Use tmp variable to avoid a -Wsign-compare warning

lib/chkname.c: Use tmp variable to avoid a -Wsign-compare warning

I used size_t because:

sysconf(3) can return -1 if the value is not supported, but then it can
only mean that there's no limit.  Having no limit is the same as having
a limit of SIZE_MAX (to which -1 is converted).

Signed-off-by: Alejandro Colomar <alx@kernel.org>
Cherry-picked-from: 6be85b0bafb5 ("lib/chkname.c: Use tmp variable to avoid a -Wsign-compare warning")
[alx: This is to cherry-pick the next commit without conflict]
Link: <https://github.com/shadow-maint/shadow/pull/801>
Link: <https://github.com/shadow-maint/shadow/pull/935>
Cc: Serge Hallyn <serge@hallyn.com>
Cc: Tobias Stoeckmann <tobias@stoeckmann.org>
Signed-off-by: Alejandro Colomar <alx@kernel.org>