fix for own error with unwrap selection checking uvedit_have_selection() failed with no UV layer.
also add some safety checks for BM_ELEM_CD_GET_* macros.


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@53780 954f8c5b-7b00-dc11-b283-0030488c597c