fix: Fixes thread guard and cleans up incoming packet reader (#1641)

### Summary
* Fixes syntax compile error when THREADGUARD is enabled.
* Removes `int packetLength` from incoming packet handles since they aren't needed.

### Developer Notes
Incoming packet handler `SpanReader` is now properly scoped to that packet by length.

Projects/UOContent/Network/ProtocolExtensions.cs

@@ -35,7 +35,7 @@ namespace Server.Network
             return packetHandlers;
         }
 
-        private static unsafe void DecodeBundledPacket(NetState state, SpanReader reader, int packetLength)
+        private static unsafe void DecodeBundledPacket(NetState state, SpanReader reader)
         {
             int cmd = reader.ReadByte();
 
@@ -57,7 +57,7 @@ namespace Server.Network
             }
             else
             {
-                ph.OnReceive(state, reader, packetLength);
+                ph.OnReceive(state, reader);
             }
         }
     }