implement a timer_list check before we return into the select() loop

this helps us to debug timer_list corruption
diff --git a/openbsc/src/select.c b/openbsc/src/select.c
index 7f45426..c11f3a5 100644
--- a/openbsc/src/select.c
+++ b/openbsc/src/select.c
@@ -78,6 +78,8 @@
 			FD_SET(ufd->fd, &exceptset);
 	}
 
+	bsc_timer_check();
+
 	if (!polling)
 		bsc_prepare_timers();
 	rc = select(maxfd+1, &readset, &writeset, &exceptset, polling ? &no_time : bsc_nearest_timer());