fix(fzf): ensure fzf_entry.pos is non-nil in on_update (#98)

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

lua/dropbar/menu.lua

@@ -1038,7 +1038,7 @@ function dropbar_menu_t:fuzzy_find_open(opts)
         end
       end
       for i, fzf_entry in ipairs(fzf_state.entries) do
-        if fzf_entry.score >= 2 then
+        if fzf_entry.score >= 2 and fzf_entry.pos then
           for _, pos_idx in ipairs(fzf_entry.pos) do
             local pos = fzf_entry.locations[pos_idx]
             vim.api.nvim_buf_set_extmark(self.buf, ns_id, i - 1, pos - 1, {