invite: Fix validation of referred_by field.

Previously, we could 500 if an organization administrator scanned
possible PreregistrationUser IDs looking for a valid invitation they
can interact with.

They couldn't do anything, so no security issue, but this fixes that
case to just be a 400 error as it should be.

zerver/lib/actions.py

@@ -4957,6 +4957,10 @@ def do_revoke_multi_use_invite(multiuse_invite: MultiuseInvite) -> None:
     notify_invites_changed(multiuse_invite.referred_by)
 
 def do_resend_user_invite_email(prereg_user: PreregistrationUser) -> int:
+    # These are two structurally for the caller's code path.
+    assert prereg_user.referred_by is not None
+    assert prereg_user.realm is not None
+
     check_invite_limit(prereg_user.referred_by.realm, 1)
 
     prereg_user.invited_at = timezone_now()