shell: character clearing in the cursor up/down handler

Hi,

In shell.c, when the cursor up/down is entered, clear_prompt will be executed.
```c
if (!__echo) {
  clear_prompt(count + 4);
} else {
  clear_prompt(count);
}
```

When `__echo` is off, `count+4` characters are cleared, which will cause PROMPT to also be cleared.
I think only `count` characters need to be cleared. Are there any special considerations?

Thank you.