OS X g++ warned that "control may reach end of non-void function" in
wxRichTextHelpInfo::ShowHelp(). This wasn't really the case but change the
function logic slightly to (hopefully) suppress the warning.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68111 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775