paging: Do not page when there is no OML link

Avoid printing error messages when paging while the BTS is down. In
case a BTS is going down it is best to just let the timers expire
normally. We can not expire them right away as multiple BTS could be
paged and we do not want to interfere with that. There is no need to
stop the queue right now.
diff --git a/openbsc/src/libbsc/paging.c b/openbsc/src/libbsc/paging.c
index fe60e3f..b731dbc 100644
--- a/openbsc/src/libbsc/paging.c
+++ b/openbsc/src/libbsc/paging.c
@@ -73,6 +73,10 @@
 	unsigned int page_group;
 	struct gsm_bts *bts = request->bts;
 
+	/* the bts is down.. we will just wait for the paging to expire */
+	if (!bts->oml_link)
+		return;
+
 	LOGP(DPAG, LOGL_INFO, "Going to send paging commands: imsi: '%s' tmsi: '0x%x'\n",
 		request->subscr->imsi, request->subscr->tmsi);