plugin treeview: fix crash

When the max_project_files is set to a higher value than the allowed
system maximum file descriptors, and opening a project directory that
causes dirmonitor to open a watch on a lot of files or directories, at
least on MacOSX it causes all system.* file functions to return nil
(for too many opened files) which breaks the project files scan.

data/plugins/treeview.lua

@@ -153,6 +153,7 @@ function TreeView:each_item()
       count_lines = count_lines + 1
       y = y + h
       local i = 1
+      if dir.files then -- if consumed max sys file descriptors this can be nil
         while i <= #dir.files and dir_cached.expanded do
           local item = dir.files[i]
           local cached = self:get_cached(dir, item, dir.name)
@@ -175,6 +176,7 @@ function TreeView:each_item()
             end
           end
         end -- while files
+      end
     end -- for directories
     self.count_lines = count_lines
   end)