logind: fix session_activate(vtnr = 0)

VT numbers start with 1. If a session has vtnr == 0, we must not assume it
is running on a VT.
Note that this could trigger the assert() below as CreateSession() sets
vtnr to 0, not <0.

src/login/logind-session.c

@@ -363,7 +363,7 @@ int session_activate(Session *s) {
         assert(s);
         assert(s->user);
 
-        if (s->vtnr < 0)
+        if (s->vtnr <= 0)
                 return -ENOTSUP;
 
         if (!s->seat)