Stop svnmucc from crashing on the new plaintext prompt warning.
Stop svnmucc from crashing on the new plaintext prompt warning.

* subversion/libsvn_subr/prompt.c
  (svn_cmdline_auth_plaintext_prompt): Don't blindly dereference the
    prompt baton; it could be NULL.


git-svn-id: f8a4e5e023278da1e04e203c7fe051e3c4285d88@871755 13f79535-47bb-0310-9956-ffa450edef68

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