Fix a fatal bug in r1543805 -- uninitialized variable.

* subversion/svn/svn.c,
  subversion/svnadmin/svnadmin.c
  (main): Initialize 'exit_code' because it's optional for sub_main() to set
    it.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1543814 13f79535-47bb-0310-9956-ffa450edef68