Fix a segfault when trying to follow the history of a file and we traverse a revision that has no changes in it.  E.G.:
Fix a segfault when trying to follow the history of a file and we traverse a
revision that has no changes in it.  E.G.:

svn diff -r3078:3079 http://svn.collab.net/repos/svn

Bug found by Øyvind A. Holm

* subversion/libsvn_client/ra.c
  (log_receiver): When changed_paths is NULL, nothing to do.

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