Minor merge sanity-check logic re-org.
Minor merge sanity-check logic re-org.

* subversion/libsvn_client/merge.c
  (do_merge): Group some related sanity checks, and reorder them so
    that error conditions are checked before no-op requests (to
    discourage naughty callers).


git-svn-id: f8a4e5e023278da1e04e203c7fe051e3c4285d88@869933 13f79535-47bb-0310-9956-ffa450edef68

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