fix: check if bufdata.last_win != nil (#356)

When opening nvim in diffview (`nvim -c DiffviewOpen`) and then toggeling aerial,
aerial errors when trying to call `vim.api.nvim_win_is_valid(bufdata.last_win)`
as bufdata.last_win == nil. This change adds a check and returns in case of a nil
value.

Co-authored-by: Linus Boehm <linusboehm@gmail.com>

lua/aerial/window.lua

@@ -538,6 +538,9 @@ M.center_symbol_in_view = function(buffer)
     return
   end
   local bufdata = data.get_or_create(bufnr)
+  if not bufdata.last_win then
+    return
+  end
   if vim.api.nvim_buf_is_valid(aer_bufnr) and vim.api.nvim_win_is_valid(bufdata.last_win) then
     local last_position = bufdata.positions[bufdata.last_win]
     if last_position then