sd-dhcp6-lease: Return only -EINVAL when a NULL lease is supplied

Suppyling a NULL lease is not a condition that needs to be asserted,
returning -EINVAL is informative enough to the caller. This simplifies
calling code and doesn't falsely indicate that something erroneous was
attempted.

src/libsystemd-network/sd-dhcp6-lease.c

@@ -110,9 +110,11 @@ int dhcp6_lease_set_preference(sd_dhcp6_lease *lease, uint8_t preference) {
 }
 
 int dhcp6_lease_get_preference(sd_dhcp6_lease *lease, uint8_t *preference) {
-        assert_return(lease, -EINVAL);
         assert_return(preference, -EINVAL);
 
+        if (!lease)
+                return -EINVAL;
+
         *preference = lease->preference;
 
         return 0;