Error if neither -iface or -name is specified

This prevents a nil pointer dereference if (e.g.) weaver is
invoked with no arguments.

Signed-off-by: Alex Bligh <alex@alex.org.uk>

weaver/main.go

@@ -92,6 +92,9 @@ func main() {
 	}
 
 	if routerName == "" {
+		if iface == nil {
+			log.Fatal("Either an interface must be specified with -iface or a name with -name")
+		}
 		routerName = iface.HardwareAddr.String()
 	}