Windows: AFSCreate avoid race leading to NULL dereference

If a test for NULL is performed ahead of an assignment and then
use of the assigned value, there is a race which can result in
the assigned value being NULL if the value being assigned is
altered by another thread.

Perform the assignment first then test based upon that.

Change-Id: I6d50619dab168c2aa12542b14217779f1be08ee9
Reviewed-on: http://gerrit.openafs.org/10296
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>