Don't throw an exception in get4() since it's actually a useful function

src/lib/dhcpsrv/radius_host_data_source.cc

@@ -271,8 +271,8 @@ RadiusHostDataSource::get4(const SubnetID& subnet_id,
 ConstHostPtr
 RadiusHostDataSource::get4(const SubnetID& subnet_id,
                           const asiolink::IOAddress& address) const {
-    // Not relevant for Radius
-    isc_throw(NotImplemented, "RadiusHostDataSource::get4 not implemented");
+    // We always assume that there is no conflict between reserved
+    // addresses and dynamic addresses, so just return nothing here.
     return (ConstHostPtr());
 }