Correctly retain message buffer for LoginPluginMessage.

Fixes #407

proxy/src/main/java/com/velocitypowered/proxy/protocol/packet/LoginPluginMessage.java

@@ -49,7 +49,7 @@ public class LoginPluginMessage extends DeferredByteBufHolder implements Minecra
     this.id = ProtocolUtils.readVarInt(buf);
     this.channel = ProtocolUtils.readString(buf);
     if (buf.isReadable()) {
-      this.replace(buf.readSlice(buf.readableBytes()));
+      this.replace(buf.readRetainedSlice(buf.readableBytes()));
     } else {
       this.replace(Unpooled.EMPTY_BUFFER);
     }