gdatetime: Widen a variable before multiplication

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