Fix missing argument validation on last option

Normally, omitting an argument to a switch yields in a `OptError("Expected switch argument")`. Example code

    var OptionParser = require('optparse').OptionParser;

    var parser = new OptionParser([['-e', '--example STRING']]);
    parser.parse(['--example', '--test']);

This does as you would expect and throws. However, calling `parser.parse(['--example']);` will not throw even though the argument to `example` is mandatory and missing. This is because the `arg` is `undefined` so neither `LONG_SWITCH_RE` nor `SHORT_SWITCH_RE` match it.