Following up on r1803144, fix an oversight in the txdelta_next_window adapter used when importing files over HTTP.
Following up on r1803144, fix an oversight in the txdelta_next_window
adapter used when importing files over HTTP.

This bug has inadvertently disabled the checksum validation for such
imports, because the resulting checksum has always been all-zeroes.

* subversion/libsvn_client/import.c
  (txdelta_next_window): Check if the svn_txdelta_window_t pointer
   is null, instead of checking that for the pointer to a pointer, which
   is never null.


git-svn-id: f8a4e5e023278da1e04e203c7fe051e3c4285d88@1804672 13f79535-47bb-0310-9956-ffa450edef68

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