Ensure that the the base_revision member is set when initializing the 
svn_client__mtcc_t struct.

* subversion/libsvn_client/mtcc.c
  (svn_client__mtcc_create): Make sure base_revision member is set in all
                             code paths, restructure things to make this a
                             bit more obvious.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1646611 13f79535-47bb-0310-9956-ffa450edef68