fix(fzf): ensure `fzf_entry.pos` is non-nil in `on_update`

[willothy]

problem: since a large portion of on_update is inside of a vim.schedule closure - if all input buffer text is removed quickly, fzf entries can have their pos field removed before the vim.schedule callback executes, causing an error. solution: add a check to ensure that fzf_entry.pos is not nil before iterating and setting extmarks

fixes #97

[Bekaboo]

Thx!