osmo-bsc: Add missing return statement causing CIPH MODE REJ

due to a missing return statement, we ran into the 'reject' case
of bssmap_handle_cipher_mode().  Due to another bug in libosmocore,
the reject message was corrupted (fixed in libosmocore commit
0c83670a595a278b7d1fb7b21b2eacab84d3c031)

openbsc/src/osmo-bsc/osmo_bsc_bssap.c

@@ -276,6 +276,8 @@ static int bssmap_handle_cipher_mode(struct osmo_bsc_sccp_con *conn,
 		goto reject;
 	}
 
+	return 0;
+
 reject:
 	resp = gsm0808_create_cipher_reject(reject_cause);
 	if (!resp) {