Return error if get unexpected 'X-SVN-VR-Base' HTTP header value instead of crashing.
Return error if get unexpected 'X-SVN-VR-Base' HTTP header value instead of
crashing.

Found by: TortoiseSVN crash dump

* subversion/libsvn_ra_serf/update.c
  (handle_fetch): Check FETCH_CTX->DELTA_BASE for NULL before comparing
   it to received header value.


git-svn-id: f8a4e5e023278da1e04e203c7fe051e3c4285d88@1702288 13f79535-47bb-0310-9956-ffa450edef68

YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion