Check buffer validity before loading Treesitter parser

[TyceHerrman]

If a buffer is wiped before asynchronous parser installation finishes, the attachment callback receives an invalid buffer number. Check validity before loading the parser (i.e., calling vim.treesitter.language.add() ) so the attachment callback returns without loading a parser for an invalid buffer, avoiding unnecessary work.

This change moves the existing guard earlier. Valid buffer attachment behavior stays the same.

[oriori1703]

This seems to be the initial place for the check in bd5406e, but it was changed in the next commit (3711862) to be how it is now.

@ollien do you remember what was your reason for changing the order?

[ollien]

@oriori1703 I don't think I had a reason grounded in evidence. I was mostly concerned with the install path, and it made sense to me to still load the parser. Thinking on it though, maybe this was misguided.

[oriori1703]

@oriori1703 I don't think I had a reason grounded in evidence. I was mostly concerned with the install path, and it made sense to me to still load the parser. Thinking on it though, maybe this was misguided.

sounds good enough for me to merge this :)