lib/: Saturate addition to avoid overflow

lib/: Saturate addition to avoid overflow

Very large values in /etc/shadow could lead to overflows.  Make sure
that these calculations are saturated at LONG_MAX.  Since entries are
based on days and not seconds since epoch, saturating won't hurt anyone.

Co-developed-by: Tobias Stoeckmann <tobias@stoeckmann.org>
Co-developed-by: Alejandro Colomar <alx@kernel.org>
Signed-off-by: Alejandro Colomar <alx@kernel.org>
Cherry-picked-from: 674409e2265e ("lib/: Saturate addition to avoid overflow")
Cc: "Serge E. Hallyn" <serge@hallyn.com>
Link: <https://github.com/shadow-maint/shadow/pull/876>
Signed-off-by: Alejandro Colomar <alx@kernel.org>