Logging: Fixed a bug in log_close_all().

Logging: Fixed a bug in log_close_all().

Under some circumstances (involving loops in substream structure),
log_close_all() crashed, because a stream could have become freed
when removing its substreams.

I have changed log_close_all() to unlink all substreams first and
then proceed with deallocating memory.

The same problem can never occur in simple log_close_stream(),
because a stream that is a part of a loop never gets a use count of 0.