Fix segfault in waiting and bug in update_wc_mergeinfo().
Fix segfault in waiting and bug in update_wc_mergeinfo().

* subversion/libsvn_client/merge.c
  (update_wc_mergeinfo): Last argument to apr_pstrcat() must be NULL.
  Don't build a rel_path with "//".

git-svn-id: f8a4e5e023278da1e04e203c7fe051e3c4285d88@865557 13f79535-47bb-0310-9956-ffa450edef68

wbond/subversion,YueLinHo/Subversion,wbond/subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,wbond/subversion,wbond/subversion,YueLinHo/Subversion,wbond/subversion,YueLinHo/Subversion,wbond/subversion,wbond/subversion