NioDatagramChannel invalid usage of internalNioBuffer

Motivation:
NioDatagramChannel attempts to unpack a AddressedEnvelope and unconditionally uses internalNioBuffer. However if the ByteBuf is a CompositeByteBuf with more than 1 components, the write will fail and throw an exception.

Modifications:
- NioDatagramChannel should check the nioBufferCount before attempting
to use internalNioBuffer

Result:
No more failure to write UDP packets on NIO when a CompositeByteBuf is
used.

transport/src/main/java/io/netty/channel/socket/nio/NioDatagramChannel.java

@@ -286,7 +286,8 @@ public final class NioDatagramChannel
             return true;
         }
 
-        final ByteBuffer nioData = data.internalNioBuffer(data.readerIndex(), dataLen);
+        final ByteBuffer nioData = data.nioBufferCount() == 1 ? data.internalNioBuffer(data.readerIndex(), dataLen)
+                                                              : data.nioBuffer(data.readerIndex(), dataLen);
         final int writtenBytes;
         if (remoteAddress != null) {
             writtenBytes = javaChannel().send(nioData, remoteAddress);