menubar.utils: Fix for invalid utf8 file names

The previous commit made this code handle invalid directories correctly.
However, it was still possible that we come across invalid file names
for which :get_path() returns nil and then we assumed this was a file
name.

Fix this by silently ignoring such files.

Signed-off-by: Uli Schlachter <psychon@znc.in>

lib/menubar/utils.lua

@@ -272,7 +272,7 @@ function utils.parse_dir(dir_path, callback)
                 local file_child = enum:get_child(info)
                 if file_type == 'REGULAR' then
                     local path = file_child:get_path()
-                    local program = utils.parse_desktop_file(path)
+                    local program = path and utils.parse_desktop_file(path)
                     if program then
                         table.insert(programs, program)
                     end