Fixed infinite loop in powernet rebuild.

If a /obj/machinery/power wasn't anchored, it wouldn't get a new list of connections, so would endlessly loop on the old list, of which that object was the first entry, ensuring that the loop would never terminate.

code/modules/power/power.dm

@@ -186,6 +186,9 @@
 			M.powernet = PN
 			P = M.get_connections()
 
+		else
+			return
+
 		if(P.len == 0)	return
 
 		O = P[1]