b2b_sdp_demux: fix possible invalid memory access

Prevent free-then-use error when b2b_key cannot be copied in shm.

Thanks go to Suchi Sahoo from Five9 for reporting it

modules/b2b_sdp_demux/b2b_sdp_demux.c

@@ -1645,8 +1645,8 @@ static int b2b_sdp_demux_start(struct sip_msg *msg, str *uri,
 		if (shm_str_dup(&client->b2b_key, b2b_key) < 0) {
 			LM_ERR("could not copy b2b client key\n");
 			/* key is not yet stored, but INVITE sent - terminate it */
-			pkg_free(b2b_key);
 			b2b_sdp_client_terminate(client, b2b_key);
+			pkg_free(b2b_key);
 			return -1;
 		}
 		pkg_free(b2b_key);