cmd/vet: fix unnamed FP check

While we're here, silence a print about failure to import fmt.
When it prints, it looks like a problem, and in fact the result
is never used in vet.

Change-Id: I76121b335026a2b09637608b997517be94fd167c
Reviewed-on: https://go-review.googlesource.com/5573
Reviewed-by: Rob Pike <r@golang.org>
2 files changed