Otherwise it could possibly overflow on 32-bit machines if `year` is
high enough, although I don’t think that’s possible because of limits
applied on it by callers. This should shut Coverity up though.

The limits applied by callers could be circumvented by calling (say)
`g_date_time_add_years()` multiple times. That’s a bug, but not one I’m
going to fix today.

Coverity CID: #1159479
Signed-off-by: Philip Withnall <withnall@endlessm.com>