Correct signed/unsigned comparison in wxGridBagSizer code.

Correct signed/unsigned comparison in wxGridBagSizer code.

Casting a signed value to unsigned type is a recipe for disaster if it
actually turns out to be negative because the comparison remains always false
and the loop becomes practically infinite. So cast the unsigned value to
signed int instead, this should be perfectly safe as the number of columns or
rows in a sizer can't exceed INT_MAX anyhow.

Notice that after the changes of the previous revision the signed value
should actually be always positive so this change is not strictly needed but
it is still safer to write the comparison like this.

See #12934.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66965 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775