Fix assert when constructing invalid wxGBSpan inside wxWidgets itself.

Fix assert when constructing invalid wxGBSpan inside wxWidgets itself.

Various wxGridBagSizer methods returning wxGBSpan asserted when trying to
construct an invalid wxGBSpan as its components must now (since r66964) be
positive.

Fix this by adding a special new method to wxGBSpan for constructing such
invalid spans and use it now.

Closes #15124.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73731 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775