Update common/logisticspipes/routing/ServerRouter.java

extra code to prevent potential NPE on pipe, if 2 pipes are removed at the same time.
fixes potential get-router-before-ready error: 
just a spin lock, relying on the main thread to tick the pipe to release the routing thread.
2013-01-26 06:54:17 [INFO] [STDOUT] Hey, don't get routers for pipes that aren't ready
2013-01-26 06:54:17 [INFO] [STDERR] java.lang.Throwable
2013-01-26 06:54:17 [INFO] [STDERR]     at logisticspipes.ticks.RoutingTableUpdateThread.run(RoutingTableUpdateThread.java:64)
2013-01-26 06:54:17 [INFO] [STDOUT] Hey, don't get routers for pipes that aren't ready
2013-01-26 06:54:17 [INFO] [STDERR] java.lang.Throwable
2013-01-26 06:54:17 [INFO] [STDERR]     at logisticspipes.pipes.basic.CoreRoutedPipe.getRouter(CoreRoutedPipe.java:432)
2013-01-26 06:54:17 [INFO] [STDERR]     at logisticspipes.pipes.basic.CoreRoutedPipe.getRouter(CoreRoutedPipe.java:447)
2013-01-26 06:54:17 [INFO] [STDERR]     at logisticspipes.routing.ServerRouter.getRouter(ServerRouter.java:694)
2013-01-26 06:54:17 [INFO] [STDERR]     at logisticspipes.routing.ServerRouter.CreateRouteTable(ServerRouter.java:387)
2013-01-26 06:54:17 [INFO] [STDERR]     at logisticspipes.routing.ServerRouter.access$000(ServerRouter.java:49)
2013-01-26 06:54:17 [INFO] [STDERR]     at logisticspipes.routing.ServerRouter$RoutingUpdateThread.run(ServerRouter.java:76)
2013-01-26 06:54:17 [INFO] [STDERR]     at logisticspipes.ticks.RoutingTableUpdateThread.run(RoutingTableUpdateThread.java:64)