Fix infinite loop in wxGrid::PosToEdgeOfLine().

Fix infinite loop in wxGrid::PosToEdgeOfLine().

Return -1 from wxGridRowOperations::GetLineBefore(0) to ensure that we exit
the loop in wxGrid::PosToEdgeOfLine(). Doing this is the logical thing to do
as wxGridColumnOperations::GetLineBefore() already behaved like this and this
was is more expected than returning 0, as the function previously did for some
reason.

Closes #15035.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73487 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775