lib/platform/linux/platform.c: polling thread accesses indication queue variables without mutex held

[jkivilin]

While doing upgrade from earlier WPC API library to c-mesh-api/master, we noticed this issue during code review.

In lib/platform/linux/platform.c, dispatch queue accesses indication queue with m_queue_mutex held. However, polling thread is accessing indication queue variables without taking mutex, for example at https://github.com/wirepas/c-mesh-api/blob/master/lib/platform/linux/platform.c#L177