fix(select): crash if initial isNaN (ex: string)

[0x0a0d]

var prompts = require("prompts")
prompts({
  type: 'select',
// initial greater than choices's size or initial is string will crash
  initial: 9,
  message: 'Hello world',
  choices: [
    {title: 'Python', value: 'py'},
    {title: 'C++', value: 'cpp'},
    {title: 'NodeJS', value: 'js'},
  ]
}).then(console.log)

crash case