Fix problem when opening project's module document

It wasn't fine to call core.open_doc without filename argument
and later call Doc:save without providing both the filename and
the absolute filename. It was giving a Doc in an inconsistent
status where self.filename was set but not self.abs_filename.
Added an asset to detect early the problem if ever happens again.

In turn the problem prevented the project's module hook to work if the
file was newly created.

data/core/commands/core.lua

@@ -141,14 +141,9 @@ command.add(nil, {
   end,
 
   ["core:open-project-module"] = function()
-    local filename = ".lite_project.lua"
-    if system.get_file_info(filename) then
-      core.root_view:open_doc(core.open_doc(filename))
-    else
-      local doc = core.open_doc()
-      core.root_view:open_doc(doc)
-      doc:save(filename)
-    end
+    local doc = core.open_doc(".lite_project.lua")
+    core.root_view:open_doc(doc)
+    doc:save()
   end,
 
   ["core:change-project-folder"] = function()

data/core/doc/init.lua

@@ -85,6 +85,8 @@ function Doc:save(filename, abs_filename)
     assert(self.filename, "no filename set to default to")
     filename = self.filename
     abs_filename = self.abs_filename
+  else
+    assert(self.filename or abs_filename, "calling save on unnamed doc without absolute path")
   end
   local fp = assert( io.open(filename, "wb") )
   for _, line in ipairs(self.lines) do