Allow the jniwrapper global reference wrappers to work with null
objects or classes.

* subversion/bindings/javahl/native/jniwrapper/jni_globalref.hpp
  (GlobalObject::GlobalObject): Create global refs more carefully.
* subversion/bindings/javahl/native/jniwrapper/jni_base.cpp
  (GlobalObject::~GlobalObject): Do not try to delete null references.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1545922 13f79535-47bb-0310-9956-ffa450edef68