library_sockfs.js can access properties of undefined `dest` (logic flaw)

> [!NOTE]
> This issue was unearthed by us due to what seems an issue on our side. My colleague @Faless explains it on the [Godot Dev Chat](https://chat.godotengine.org/channel/web?msg=oMN3SjGjhpyzZqbw6). Though, the issue underlined is still a logic flaw.

Regression introduced by #22630

https://github.com/emscripten-core/emscripten/blob/06cebfca51f908d3bc6897113330e45a77010db2/src/library_sockfs.js#L632-L647

If `dest == null` and `sock.type !== {{{ cDefs.SOCK_DGRAM }}}` (see code above), then the following error occurs: