Abort if processing SCTP connection without HNB context

It was observed that under some circumstances (after HNBAP
HNB-De-Register) we end up crashing because a connection has no HNB
assigned to it. Let's explicitly assert if that happens, in order
clarify and avoid same sort of thing happening without clear view on
what's going on.
The issue will be fixed in a follow-up patch.

Closes: OS#5676
Change-Id: I1eedab6f3ac974e942b02eaae41556f87dd8b6ba

src/osmo-hnbgw/hnbgw.c

@@ -245,6 +245,7 @@ static int hnb_read_cb(struct osmo_stream_srv *conn)
 	if (!msg)
 		return -ENOMEM;
 
+	OSMO_ASSERT(hnb);
 	/* we store a reference to the HomeNodeB in the msg->dest for the
 	 * benefit of various downstream processing functions */
 	msg->dst = hnb;