Fix error iterating nil object when mapping functions to line numbers (#320)

ldoc.lua

@@ -573,12 +573,14 @@ if ldoc.prettify_files then
    for F in file_list:iter() do
       files:append(F.filename)
       local mod = F.modules[1]
+      if mod then
         local ls = List()
         for item in mod.items:iter() do
            ls:append(item.lineno)
         end
         linemap[F.filename] = ls
       end
+   end
 
    if type(ldoc.prettify_files) == 'table' then
       files = tools.expand_file_list(ldoc.prettify_files, '*.*')