fill now expects a fillable buffer + resets the buffer's position when nothing is read

Signed-off-by: Ludovic Orban <lorban@bitronix.be>

jetty-http3/http3-server/src/main/java/org/eclipse/jetty/http3/server/ServerDatagramEndPoint.java

@@ -107,13 +107,12 @@ public class ServerDatagramEndPoint extends IdleTimeout implements EndPoint, Man
     @Override
     public int fill(ByteBuffer buffer) throws IOException
     {
-        BufferUtil.flipToFill(buffer);
         int headerPosition = buffer.position();
         buffer.position(buffer.position() + ENCODED_ADDRESS_LENGTH);
         InetSocketAddress peer = (InetSocketAddress)channel.receive(buffer);
         if (peer == null)
         {
-            buffer.position(0);
+            buffer.position(headerPosition);
             buffer.flip();
             return 0;
         }