gsm_subscriber_base: Pass the subscriber as param to avoid crash.

The function we call will take the item from the list and do
the right thing.
diff --git a/openbsc/src/gsm_subscriber_base.c b/openbsc/src/gsm_subscriber_base.c
index 40c3bbd..c0d1261 100644
--- a/openbsc/src/gsm_subscriber_base.c
+++ b/openbsc/src/gsm_subscriber_base.c
@@ -106,7 +106,7 @@
 	/* paging failed, quit now */
 	if (rc <= 0) {
 		subscr_paging_cb(GSM_HOOK_RR_PAGING, GSM_PAGING_EXPIRED,
-				 NULL, NULL, request->param);
+				 NULL, NULL, subscr);
 	}
 }