ftl library crashes in FTL_LOG enqueue_status_msg

[andrewsml]

this happen when internet connection breaks or gets much weak.
crash can be reproduced using XSplit Broadcaster.
Steps:

1. Start FTL stream from XSplit Broadcaster to Mixer.
2. Limit XSplit Broadcaster upload speed to 1kbps using NetBalancer software.
3. Stop FTL stream.

I debugged this problem and found that ftl->status_q.mutex
was deleted by ftl_ingest_disconnect function, while some threads like ping_thread
are still alive and continue calling FTL_LOG with deleted ftl->status_q.mutex.

[tyiki-badwell]

This issue occurs when the application does not call ftl_ingest_disconnect before calling ftl_ingest_destroy.
It seems to be difficult to care.