hnbgw_rx_hnb_deregister: Don't call hnb_context_release()

Don't release the HNB context as there's plenty of code that
assumes there's always a HNB context associated with a SCTP connection.

Instead, simply unset the hnb_registered flag in the context when
processing a HNB_DE-REGISTER.

Related: OS#5676

Change-Id: Id5c4f5c900ea049f54afbf58edb84b4dc00b1dcb

src/osmo-hnbgw/hnbgw_hnbap.c

@@ -388,7 +388,7 @@ static int hnbgw_rx_hnb_deregister(struct hnb_context *ctx, ANY_t *in)
 	LOGHNB(ctx, DHNBAP, LOGL_DEBUG, "HNB-DE-REGISTER cause=%s\n", hnbap_cause_str(&ies.cause));
 
 	hnbap_free_hnbde_registeries(&ies);
-	hnb_context_release(ctx);
+	ctx->hnb_registered = false;
 
 	return 0;
 }