Assist proper thread synchronization when stopping the logger thread.

I was finding that on my dev box, occasionally attempting to "stop now" in
trunk would cause Asterisk to hang. I traced this to the fact that the logger
thread was waiting on a condition which had already been signalled. The logger
thread also need to be sure to check the value of the close_logger_thread variable.

The close_logger_thread variable is only checked when the list of logmessages is empty.
This allows for the logger thread to print and free any pending messages before exiting.



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@176174 65c4cc65-6c06-0410-ace0-fbb531ad65f3