Well you don't need that line at all.
The ExecuteScalar method returns a single value eg the number of records affected in an update query.
In your case it holds the id of the user which is an authenticated user.
If he is not, the ExecuteScalar will return nothing.
So what you could do id this.
Dim strid as string 'Assuming your id is a string value
strid=dbcomm.ExecuteScalar()
If not strid is nothing Then
System.Web.Security.FormsAuthentication.RedirectFromLoginPage(strid, False)
End If
Hope its clear.