paging_request() now returns the number of started paging requests

this helps the caller to determine if he will ever get called back
or not (and if he should free his data structures now or not)
diff --git a/openbsc/src/gsm_04_11.c b/openbsc/src/gsm_04_11.c
index a7f69d1..277a321 100644
--- a/openbsc/src/gsm_04_11.c
+++ b/openbsc/src/gsm_04_11.c
@@ -1046,7 +1046,7 @@
 	/* if not, we have to start paging */
 	rc = paging_request(subscr->net, subscr, RSL_CHANNEED_SDCCH,
 			    paging_cb_send_sms, sms);
-	if (rc < 0)
+	if (rc <= 0)
 		sms_free(sms);
 
 	return 0;