Don't attempt to decode empty buffers (fixes #482)

Stuff like this makes me want to drop support for <=1.13 versions of Minecraft.

proxy/src/main/java/com/velocitypowered/proxy/protocol/netty/MinecraftDecoder.java

@@ -62,7 +62,7 @@ public class MinecraftDecoder extends ChannelInboundHandlerAdapter {
   }
 
   private void tryDecode(ChannelHandlerContext ctx, ByteBuf buf) throws Exception {
-    if (!ctx.channel().isActive()) {
+    if (!ctx.channel().isActive() || !buf.isReadable()) {
       buf.release();
       return;
     }