shada: Fix non-writeable ShaDa directory handling

Before this change,

    nvim -i /etc/shada

segfaults on exit if the file does not exist and user does not have
permissions to create the file at /etc/shada.

Closes #5296
Reported in #5277
https://github.com/neovim/neovim/issues/5277#issuecomment-243937255

test/functional/shada/helpers.lua

@@ -8,8 +8,8 @@ local mpack = require('mpack')
 local tmpname = helpers.tmpname()
 local additional_cmd = ''
 
-local function nvim_argv()
-  local argv = {nvim_prog, '-u', 'NONE', '-i', tmpname, '-N',
+local function nvim_argv(shada_file)
+  local argv = {nvim_prog, '-u', 'NONE', '-i', shada_file or tmpname, '-N',
                 '--cmd', 'set shortmess+=I background=light noswapfile',
                 '--cmd', additional_cmd,
                 '--embed'}
@@ -20,8 +20,8 @@ local function nvim_argv()
   end
 end
 
-local reset = function()
-  set_session(spawn(nvim_argv()))
+local reset = function(shada_file)
+  set_session(spawn(nvim_argv(shada_file)))
   meths.set_var('tmpname', tmpname)
 end