Protect against possible NULL dereference after unsubscribing.

src/read_handle_server.c

@@ -441,6 +441,7 @@ int mqtt3_handle_connect(struct mosquitto_db *db, struct mosquitto *context)
 			found_context->sub_count = 0;
 
 			for(i=0; i<context->sub_count; i++){
+				if(context->subs[i]){
 					leaf = context->subs[i]->subs;
 					while(leaf){
 						if(leaf->context == found_context){
@@ -450,6 +451,7 @@ int mqtt3_handle_connect(struct mosquitto_db *db, struct mosquitto *context)
 					}
 				}
 			}
+		}
 
 		found_context->clean_session = true;
 		found_context->state = mosq_cs_disconnecting;