Prevent out of bounds array access.

src/loop.c

@@ -539,7 +539,7 @@ static void loop_handle_reads_writes(struct mosquitto_db *db, struct pollfd *pol
 				}
 			}while(SSL_DATA_PENDING(context));
 		}
-		if(pollfds[context->pollfd_index].revents & (POLLERR | POLLNVAL | POLLHUP)){
+		if(context->pollfd_index >= 0 && pollfds[context->pollfd_index].revents & (POLLERR | POLLNVAL | POLLHUP)){
 			do_disconnect(db, context);
 			continue;
 		}