Add a null check for the throwable message in NetWorkDispatcher#exceptionCaught (#3688)

(cherry picked from commit 1be1bb59c5)

src/main/java/net/minecraftforge/fml/common/network/handshake/NetworkDispatcher.java

@@ -586,7 +586,7 @@ public class NetworkDispatcher extends SimpleChannelInboundHandler<Packet<?>> im
         if (!(cause instanceof ClosedChannelException))
         {
             // Mute the reset by peer exception - it's disconnection noise
-            if (cause.getMessage().contains("Connection reset by peer"))
+            if (cause.getMessage() != null && cause.getMessage().contains("Connection reset by peer"))
             {
                 FMLLog.log(Level.DEBUG, cause, "Muted NetworkDispatcher exception");
             }