Fix cdpath commit, which was broken in several ways

This fixes commit 04681ff7ceb8a as follows:

 - check when cdpath is None. Otherwise split fails on None objects and
   causes ranger to crashes when CDPATH is not set in the environment.

 - actually set cdpath variable in the lowercase case. Before it was not
   assigned to any variable.

 - join paths with os.path.join instead of concatenating with '/'.