Loosen the assert in IsScrollIncrement().

wxWindowGTK::GTKGetScrollEventType() tries to map GtkAdjustment's delta
to either step or page increment by comparing it with the respective
GtkAdjustment steps. Both of them can be 0, but this code is not
expected to be called in such case.

Yet, in practice, it occasionally is, see e.g.
http://devel.aegisub.org/ticket/979GTKGetScrollEventType (wxWebView is
prone to it as well).

Check for >= 0 instead, to be more robust when faced with unexpected
input from GTK+.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74730 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775