Don't accept invalid values for rows/columns in wxGBSpan ctor.

Don't accept invalid values for rows/columns in wxGBSpan ctor.

wxGBSpan must have strictly positive row and column span as otherwise the grid
bag sizer code could enter an infinite loop trying to exceed a negative number
which it casted to an unsigned one. And while the cast itself is incorrect too
the program still behaves undesirably (produces a lot of asserts in debug
build and then crashes or crashes directly in release) if a zero size span is
used so it seems better to prevent this from happening.

Closes #12934.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66964 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775