Bug fix for 64 bit Windows backend of xolox#shell#execute_with_dll()

This bug fix is not actually confirmed to resolve the problem that was
reported in issue #13 (yet), however going by the documentation this is
an improvement in any case, so I will go ahead and release it and hope
it also fixes the reported problem :-)

Related to issue #13 on GitHub:
  https://github.com/xolox/vim-shell/issues/13