Prior to commit 6195adda9b the sm->user
dereference did not exist here. While this is in practice non-NULL,
better use more defensive construction here to avoid NULL pointer
dereference should this ever change.
In theory, this should not really be needed, but Windows 7 uses
Registrar mode to probe AP's WPS capabilities before trying to use
Enrollee and fails if the AP does not allow that probing to happen.
This allows the AP to start as an Enrollee and send M1, but refuse
to continue beyond that (M3 will not be sent if AP PIN is not known).
