fix: race condition when stopping loading timer (#331)

On Neovim 0.10, calling timer:stop() is not canceling the callback.
There appears to be some minimum amount of buffer time remaining in the
timer in order for it to be able to be canceled correctly. I'm getting
around this by adding a simple guard statement

lua/aerial/loading.lua

@@ -20,6 +20,9 @@ M.set_loading = function(aer_bufnr, is_loading)
         0,
         80,
         vim.schedule_wrap(function()
+          if not timers[aer_bufnr] then
+            return
+          end
           local lines = { M.spinner_frames[i + 1] .. " Loading" }
           util.render_centered_text(aer_bufnr, lines)
           i = (i + 1) % #M.spinner_frames