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 #34660

Change-Id: I56f9d21e5342d07e7d843d253267e93a29707904
Reviewed-on: https://go-review.googlesource.com/c/go/+/198460
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>