Delete logThread in logStop

Seems I never actually deleted the logThread when stopping logging.  This was
causing issues as then the timers don't get stopped (as that's in the dtor)
on quick runs, causing nasty messages, and maybe even crashes.

Refs #10867

mythtv/libs/libmythbase/logging.cpp

@@ -864,6 +864,8 @@ void logStop(void)
     {
         logThread->stop();
         logThread->wait();
+        delete logThread;
+        logThread = NULL;
     }
 }