net: check SkipAdditional error result

This will avoid a potential endless loop for a corrupt DNS packet.

For #66754

Change-Id: I46591b3f7695bcc88d2312833e45955f8c129d2b
Reviewed-on: https://go-review.googlesource.com/c/go/+/578375
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Commit-Queue: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

src/net/dnsclient_unix.go

@@ -277,7 +277,9 @@ func extractExtendedRCode(p dnsmessage.Parser, hdr dnsmessage.Header) (dnsmessag
 		if ahdr.Type == dnsmessage.TypeOPT {
 			return ahdr.ExtendedRCode(hdr.RCode), hasAdd
 		}
-		p.SkipAdditional()
+		if err := p.SkipAdditional(); err != nil {
+			return hdr.RCode, hasAdd
+		}
 	}
 }