JavaHL: Fix potential exception thrown by the copy source discovery API, which can occur when a path has no copy source.
JavaHL: Fix potential exception thrown by the copy source discovery
API, which can occur when a path has no copy source.

[ in subversion/bindings/javahl/ ]

* native/SVNClient.cpp
  (getCopySource): Return NULL when svn_client__get_copy_source()
   returns an invalid revision number (and likely also a NULL copyfrom
   path).

Found by: markphip

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion