[release-branch.go1.13] 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 #34662
Updates #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>
(cherry picked from commit
f0e940ebc985661f54d31c8d9ba31a553b87041b)
Reviewed-on: https://go-review.googlesource.com/c/go/+/198489
Reviewed-by: Michael Hendricks <michael@ndrix.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>