If you simply type "123456j" then you'll crash busybox vi

The problem is the blind recursion in do_cmd, where you recurse without bound based on 'cmdcnt'

---
	case 10:			// Newline ^J
	case 'j':			// j- goto next line, same col
	case KEYCODE_DOWN:	// cursor key Down
		if (--cmdcnt > 0) {
			do_cmd(c);
		}
		dot_next();		// go to next B-o-l
		dot = move_to_col(dot, ccol + offset);	// try stay in same col
		break;
---

several commands seem to follow this idiom and thus have the same issue.