[release-branch.go1.12] net: avoid an infinite loop in LookupAddr

If a request for a PTR record returned a response with a non-PTR
answer, goLookupPTR would loop forever.  Skipping non-PTR answers
guarantees progress through the DNS response.

Fixes #34661
Updates #34660

Change-Id: Ib5e5263243bc34b9e2f85aa2b913c9cd50dbcaa5
Reviewed-on: https://go-review.googlesource.com/c/go/+/198497


Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>