dns: consistency; performance on malicious input

This PR addresses two issues in lib/dns.js.

1. Inconsistent behavior on some invalid servers, #20441 (closed)
2. Quadratic regex, a potential performance issue, #20443 (closed)

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines