vm: reuse validateString of internal/validators

[ZYSzys]

The validateString function in vm.js is just one more judgement which validates whether prop !== undefined than the validateString function in internal/validators:

node/lib/vm.js

Lines 153 to 156 in 8f4b924

	function validateString(prop, propName) {
	if (prop !== undefined && typeof prop !== 'string')
	throw new ERR_INVALID_ARG_TYPE(propName, 'string', prop);
	}

node/lib/internal/validators.js

Lines 123 to 126 in 8f4b924

	function validateString(value, name) {
	if (typeof value !== 'string')
	throw new ERR_INVALID_ARG_TYPE(name, 'string', value);
	}

So now just reuse the general validateString in validator.js totally.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

[antsmartian]

LGTM

[antsmartian]

CI : https://ci.nodejs.org/job/node-test-pull-request/19581/ ✔️

[danbev]

Landed in e5966ef.