Change the loop condition to avoid comparing unsigned value with 0.

Change the loop condition to avoid comparing unsigned value with 0.

This resulted in (useful) g++ warning and didn't make any sense in any case.

Check for the loop variable value being 0 at the end of the loop instead now.
If the old code was correct it shouldn't change its behaviour and if not, this
might fix a bug.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68100 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775