Windows: AFSParseName always set FileName output

The FileName output parameter is used by the caller even when an
error occurs.  In case of error it indicates that path that failed
to parse. Not all of the error paths set FileName.

Start AFSParseName() with FileName referring to
IrpSp->FileObject->FileName.  It can be updated as required later.

Reviewed-on: http://gerrit.openafs.org/11395
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
(cherry picked from commit 2f4f1e3f8619b1531838a62031e969cbf64be7dd)

Change-Id: I247a3f33b76230071586f09a335bab80b7fdb10f
Reviewed-on: http://gerrit.openafs.org/11407
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>