Fix boundary check in DefaultChannelPipeline.addFirst(..)

- Thanks @normanmaurer

transport/src/main/java/io/netty/channel/DefaultChannelPipeline.java

@@ -143,7 +143,7 @@ public class DefaultChannelPipeline implements ChannelPipeline {
         if (handlers == null) {
             throw new NullPointerException("handlers");
         }
-        if (handlers[0] == null) {
+        if (handlers.length == 0 || handlers[0] == null) {
             return this;
         }