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.

Change-Id: I37c615aa815affec0c8f4dfef7d8c5777650c275
Reviewed-on: http://gerrit.openafs.org/11395
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>