dns: setServers throws inconsistent TypeError

[davisjam]

• Version

./out/Release/node  --version
v11.0.0-pre

• Platform

(20:24:36) jamie@woody ~/Desktop/floss/node $ uname -a
Linux woody 4.13.0-38-generic #43~16.04.1-Ubuntu SMP Wed Mar 14 17:48:43 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux

• Subsystem

DNS

Problematic behavior

The dns.setServers API responds inconsistently on different invalid input.

Good behavior:

dns.setServers([' '])
TypeError [ERR_INVALID_IP_ADDRESS]: Invalid IP address:  
    at servers.forEach (dns.js:319:11)
    at Array.forEach (<anonymous>)
    at Resolver.setServers (dns.js:295:11)
    at Object.defaultResolverSetServers [as setServers] (dns.js:360:12)

Bad behavior:

> dns.setServers(['\n'])
TypeError: Cannot read property 'Symbol(Symbol.iterator)' of null
    at servers.forEach (dns.js:312:27)
    at Array.forEach (<anonymous>)
    at Resolver.setServers (dns.js:295:11)
    at Object.defaultResolverSetServers [as setServers] (dns.js:360:12)

A null pointer dereference is rather less nice than an error like ERR_INVALID_IP_ADDRESS.

I believe the problem is in dns.js:

...
  servers.forEach((serv) => {
    ...
    const match = serv.match(IPv6RE);
    // we have an IPv6 in brackets
    if (match) {
       ....
    }
    ...
    const [, s, p] = serv.match(addrSplitRE);

While the IPv6 code only accesses match if there was a match, the addrSplitRE code assumes there is a match, which can lead to this unsavory TypeError.

[davisjam]

Working on a PR