Add default block to CompositeByteBuf (#11178)

Motivation:

Switch statements should always have a default block to ensure we not "fall-through" by mistake.

Modification:

Add default block

Result:

code cleanup.

Signed-off-by: xingrufei <xingrufei@sogou-inc.com>

Co-authored-by: xingrufei <xingrufei@sogou-inc.com>

buffer/src/main/java/io/netty/buffer/CompositeByteBuf.java

@@ -1659,6 +1659,9 @@ public class CompositeByteBuf extends AbstractReferenceCountedByteBuf implements
             if (buf.nioBufferCount() == 1) {
                 return buf.nioBuffer(c.idx(index), length);
             }
+            break;
+        default:
+            break;
         }
 
         ByteBuffer[] buffers = nioBuffers(index, length);