Now that checknil has only a single caller, inline it.

Passes toolstash-check.

Change-Id: I5b13596bef84dd9a3e7f4bff8560903f1e54acfb
Reviewed-on: https://go-review.googlesource.com/c/148829
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>