Fix stack overflow with more than 4 DNS servers & TLS

We were stomping beyond the end of the MAXNS-sized array in
ExternalPrivateDnsStatus. This is an emergency fix for a crash reported
in dogfood. Tests will follow later, along with a code cleanup.

Bug: 119694273
Test: builds!
Change-Id: If0f86212797e04cb8f6f35839f4b92b9c111278e