GMutex (linux): detect and report some errors

Detect the following two errors:

 - attempting to unlock a mutex that is not locked

 - attempting to clear a mutex that was not initialised or was
   initialised but is still locked

Both of these are fatal errors.  We avoid using g_error() here because
doing so would involve calls back into the GMutex code, and if things
are going off the rails then we want to avoid that.

https://bugzilla.gnome.org/show_bug.cgi?id=731986