Fix command packet wrong mark as unsigned (#810)

Dropping previousMessages will break the chat chain

proxy/src/main/java/com/velocitypowered/proxy/protocol/packet/chat/PlayerCommand.java

@@ -114,9 +114,6 @@ public class PlayerCommand implements MinecraftPacket {
     timestamp = Instant.ofEpochMilli(buf.readLong());
 
     salt = buf.readLong();
-    if (salt == 0L) {
-      unsigned = true;
-    }
 
     int mapSize = ProtocolUtils.readVarInt(buf);
     if (mapSize > MAX_NUM_ARGUMENTS) {
@@ -152,6 +149,10 @@ public class PlayerCommand implements MinecraftPacket {
       }
     }
 
+    if (salt == 0L && previousMessages.length == 0) {
+      unsigned = true;
+    }
+
   }
 
   @Override