gtk_text_iter_get_line and get_line_offset work fine on the end iter. Don't bail...

gtk_text_iter_get_line and get_line_offset work fine on the end iter. Don't bail if we are at the end of buffer.
Fixes one part of bug #1516281, the other part is controversial and probably not reasonable to change.
Also deal with NULL being passed as the x or y pointer, as is done also on wxMSW (this was done on 2.6 branch as
well, but not noted).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@40089 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775