lib/getdate.y: get_date(): Fix calculation

lib/getdate.y: get_date(): Fix calculation

Instead of adding 1, we should add the value the we stored previously in
the variable.

Fixes: 45c6603cc86c ("[svn-upgrade] Integrating new upstream version, shadow (19990709)")
Closes: <https://github.com/shadow-maint/shadow/issues/939>
Link: <https://github.com/shadow-maint/shadow/pull/942>
Reported-by: Michael Vetter <jubalh@iodoru.org>
Reported-by: Gus Kenion <https://github.com/kenion>
Cc: Iker Pedrosa <ipedrosa@redhat.com>
Cc: Serge Hallyn <serge@hallyn.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>