courier: Fix SMTP outgoing security level check

The outgoing security level checks are not being performed, because of a
bug: the courier thinks the "to"'s domain is always empty.

This patch fixes the bug by simplifying the logic, as there's no need
for the conditional (there is always a domain in the "to" address if it
got to the SMTP courier).

test/t-09-loop/run.sh

@@ -41,4 +41,12 @@ while sleep 0.1; do
 	fi
 done
 
+# Test that A has outgoing domaininfo for srv-b.
+# This is unrelated to the loop itself, but serves as an end-to-end
+# verification that outgoing domaininfo works.
+if ! grep -q "outgoing_sec_level: TLS_INSECURE" ".data-A/domaininfo/s:srv-b";
+then
+	fail "A is missing the domaininfo for srv-b"
+fi
+
 success