internal/cache: verify that timestamp is positive
internal/cache: verify that timestamp is positive

This backports the following upstream commit:

commit 5f8d8161e66bda5fa9559648f7793e96885c89e3
Author: Bryan C. Mills <bcmills@google.com>
Date:   Tue Jul 30 14:51:26 2019 -0400

    cmd/go/internal/cache: verify that timestamp is positive

    An apparent typo caused us to re-check size instead.

    Updates #29667

    Change-Id: Icc1fb9cef1ba77b8490c8fbe6c343be06ff48d8d
    Reviewed-on: https://go-review.googlesource.com/c/go/+/188020
    Run-TryBot: Bryan C. Mills <bcmills@google.com>
    TryBot-Result: Gobot Gobot <gobot@golang.org>
    Reviewed-by: Jay Conrod <jayconrod@google.com>

dominikh/go-tools,dominikh/go-tools,dominikh/go-tools,dominikh/go-tools