fix: bunch of meaningless chars in the walkthrough/source code

level9: done (wip - walkthrough)

level7/source.c

@@ -9,7 +9,7 @@ void m(void)
 {
 	time_t current_time;
 
-	current_time = time((time_t *)0x0);
+	current_time = time(NULL);
 	printf("%s - %d\n", c, current_time);
 	return;
 }