res_pjsip_session: Fixed NULL active media topology handle

Added NULL pointer check to prevent Asterisk crash.

ASTERISK-29215

Change-Id: If07e50ea8d78cb610af9195fc13b5dca4bfcef95

res/res_pjsip_session.c

@@ -2280,7 +2280,7 @@ static int sip_session_refresh(struct ast_sip_session *session,
 					ast_sip_session_get_name(session));
 			}
 
-			if (active_media_state) {
+			if (active_media_state && active_media_state->topology) {
 				struct ast_sip_session_media_state *new_pending_state;
 				/*
 				 * We need to check if the passed in active and pending states are equal