Fix the following segfault in svnrdump ('session' is NULL):
Fix the following segfault in svnrdump ('session' is NULL):

#0  0x0388a87d in svn_ra_get_latest_revnum (session=0x0, 
    latest_revnum=0xcfbddbcc, pool=0x88ada018)
    at subversion/libsvn_ra/ra_loader.c:586
586       return session->vtable->get_latest_revnum(session, latest_revnum, pool
);

There is another segfault still but let's fix them one at a time.

* subversion/svnrdump/svnrdump.c
  (open_connection): Add a docstring. Make the SESSION parameter a pointer
   to a pointer so the caller can actually get a useful result from us.
   This used to work before r965418 when SESSION was still a global variable.
  (main): Adjust caller.

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