zip: fix staticcheck errors

Reported by dmitshur in a comment in CL 202042.

Change-Id: I913bd87d9a17a9cf19ac869f8b7166b05e839758
Reviewed-on: https://go-review.googlesource.com/c/mod/+/218237
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
2 files changed