[config] Never return NULL mode setting

If usb-moded configuration is corrupted, it can lead to segfaults as the
return value from config_get_mode_setting() is not always checked.

Make config_get_mode_setting() always return a valid c-string.

Use MODE_CHARGING as fallback value, and remove any alternative fallback
strategies used elsewhere in the code base.

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

src/usb_moded-config.c

@@ -400,11 +400,20 @@ static char * config_get_kcmdline_string(const char *entry)
 
 char * config_get_mode_setting(void)
 {
-    char * mode = config_get_kcmdline_string(MODE_SETTING_KEY);
-    if (mode != NULL)
-        return(mode);
+    char *mode = 0;
 
-    return(config_get_conf_string(MODE_SETTING_ENTRY, MODE_SETTING_KEY));
+    /* Kernel command line can be used to override settings */
+    if( (mode = config_get_kcmdline_string(MODE_SETTING_KEY)) )
+        goto EXIT;
+
+    if( (mode = config_get_conf_string(MODE_SETTING_ENTRY, MODE_SETTING_KEY)) )
+        goto EXIT;
+
+    /* If no default mode is configured, treat it as charging only */
+    mode = g_strdup(MODE_CHARGING);
+
+EXIT:
+    return mode;
 }
 /*
  *  @param settingsfile: already opened settingsfile we want to read an entry from

src/usb_moded-dbus.c

@@ -417,13 +417,6 @@ static DBusHandlerResult umdbus_msg_handler(DBusConnection *const connection, DB
         {
             char *config = config_get_mode_setting();
 
-            if(!config)
-            {
-                /* Config is corrupted or we do not have a mode
-                 * configured, fallback to undefined. */
-                config = g_strdup(MODE_UNDEFINED);
-            }
-
             if((reply = dbus_message_new_method_return(msg)))
                 dbus_message_append_args (reply, DBUS_TYPE_STRING, &config, DBUS_TYPE_INVALID);
             g_free(config);