The old implementation of Jar made the assumption that the host names
in the URLs given to SetCookies() and Cookies() methods are well-formed.
This is not an unreasonable assumption as malformed host names do not
trigger calls to SetCookies or Cookies (at least not from net/http)
as the HTTP request themselves are not executed. But there can be other
invocations of these methods and at least on Linux it was possible to
make DNS lookup to domain names with two trailing dots (see issue #7122).

This is an old bug and this CL revives an old change (see
https://codereview.appspot.com/52100043) to fix the issue. The discussion
around 52100043 focused on the interplay between the jar and the
public suffix list and who is responsible for which type if domain name
canonicalization. The new bug report in issue #19384 used a nil public
suffix list which demonstrates that the package cookiejar alone exhibits
this problem and any solution cannot be fully delegated to the
implementation of the used PublicSuffixList: Package cookiejar itself
needs to protect against host names of the form ".." which triggered an
out-of-bounds error.

This CL does not address the issue of host name canonicalization and
the question who is responsible for it. This CL just prevents the
out-of-bounds error: It is a very conservative change, i.e. one might
still set and retrieve cookies for host names like "weird.stuf...".
Several more test cases document how the current code works.

Fixes #19384.

Change-Id: I14be080e8a2a0b266ced779f2aeb18841b730610
Reviewed-on: https://go-review.googlesource.com/37843
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>