fix: respect the configured receive window

[paulpach]

the count should be less than the configured receive window, not the default receive window

[miwarnec]

hey @skywind3000 can you confirm that this is a bug? we are using kcp for Mirror Networking in Unity. this bug seems to limit max message size for no obvious reason.

[miwarnec]

we have a applied this fix for C# kcp version. it works well.

我们在 C# 版本中修复了它。它工作正常。

[szhnet]

这里应该是 if (count >= 256) return -2; 才对吧，frg字段只有一个字节。

[miwarnec]

这里应该是 if (count >= 256) return -2; 才对吧，frg字段只有一个字节。

Thank you. We noticed this too just now. 谢谢你。我们刚才也注意到了这一点。

[c0nd3v]

@skywind3000 This is your project which is supposed to be a replacement for TCP (Most used protocol in the world). Why have you not merged this pull request yet.