Revert "Change sw::mark::IMark::GetOtherMarkPos return type from ref

... to pointer"

CrossRefBookmark::GetOtherMarkPos() is apparently not supposed to be
called, so just replace the undefined null-reference with a call to
abort().

This reverts commit dcf0ac048ce745774f401ada72b051757298c559.

Change-Id: I55b4accb4f42031df5671e8a30897e3aff9b56f9
avoid undefined null-reference in CrossRefBookmark::GetOtherMarkPos()

CrossRefBookmark::GetOtherMarkPos() is apparently not supposed to be
called, so just replace the undefined null-reference with a call to
abort().

Change-Id: Ie8e141d661b870573092d20bd85fa1ae04506421