bsc: Stop "show paging" from crashing before the first paging

The paging structure is still initialized lazily and we attempt
to traverse it before it is ready. The crash was discovered by
Katerina. Removing the lazy initialization will take a bit of work
as the gsm_data_shared is used by the osmo-bts and the OpenBSC but
paging is different.

openbsc/src/libbsc/bsc_vty.c

@@ -1069,6 +1069,9 @@ static void bts_paging_dump_vty(struct vty *vty, struct gsm_bts *bts)
 {
 	struct gsm_paging_request *pag;
 
+	if (!bts->paging.bts)
+		return;
+
 	llist_for_each_entry(pag, &bts->paging.pending_requests, entry)
 		paging_dump_vty(vty, pag);
 }