Add an assert for the working_url in checkout_file().

This member of the struct is allowed to be NULL for non-checked out dirs.
But checkout_file() assumes that it is always true (which it should be).
If this assumption fails at least we'll get an error this way.

* subversion/libsvn_ra_serf/commit.c
  (checkout_file): Add SVN_ERR_ASSERT on parent_dir->working_url.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1512196 13f79535-47bb-0310-9956-ffa450edef68