Avoid a "may be used uninitialized" error in JavaHL.

* subversion/bindings/javahl/native/OperationContext.cpp
  (OperationContext::getAuthBaton): Properly nullify all prompt
   functions and batons if we don't have a prompter callback.
   Before this change, one of them was left uninitialized.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1535600 13f79535-47bb-0310-9956-ffa450edef68