[dbus] Fix free() vs g_free() issues

The get_hidden_modes() returns data allocated via g_key_file_get_string()
which should be released with g_free() and fallback uses strdup() which
should use free().

Use g_strdup() in the fallback case and release memory with g_free().

Signed-off-by: Simo Piiroinen <simo.piiroinen@jollamobile.com>

src/usb_moded-dbus.c

@@ -248,10 +248,10 @@ static DBusHandlerResult msg_handler(DBusConnection *const connection, DBusMessa
         {
                 char *config = get_hidden_modes();
                 if(!config)
-                    config = strdup("");
+                    config = g_strdup("");
                 if((reply = dbus_message_new_method_return(msg)))
                         dbus_message_append_args (reply, DBUS_TYPE_STRING, &config, DBUS_TYPE_INVALID);
-                free(config);
+                g_free(config);
         }
 	else if(!strcmp(member, USB_MODE_NETWORK_SET))
 	{