Applied patch [ 650078 ] Add better checking to wxComboBox

Applied patch [ 650078 ] Add better checking to wxComboBox

This patch adds extra checks, checks to see and report
if the provided item index is valid.

An other thing:
- IMHO wxComboBox::GetSelection() was wrong, I
corrected this (now it behaves the same as wxMSW,
don't know about wxGTK and others...).

Hans Van Leemputten (hansvl)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@18142 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775