paging: paging_flush_bts: be sure pending_requests is initalized

The BSSMAP reset causes the paging requests to be flushed. When
this happens right after startup then calling paging_flush_bts()
may be called when the list bts->paging.pending_requests is not
yet initalized, which causes a segfault.

Call paging_init_if_needed() to be sure that the list is
inizalized (like the other functions also do)

Change-Id: I42ddbfdec6f9d74d858ad13cc38b5b64061d08dc

src/libbsc/paging.c

@@ -477,6 +477,8 @@ void paging_flush_bts(struct gsm_bts *bts, struct bsc_msc_data *msc)
 {
 	struct gsm_paging_request *req, *req2;
 
+	paging_init_if_needed(bts);
+
 	llist_for_each_entry_safe(req, req2, &bts->paging.pending_requests, entry) {
 		if (msc && req->msc != msc)
 			continue;