mongo: fix vet warning

The pre-push script's vet invocation, it does nothing.
If you invoke "go tool vet .", it returns rc 0 even if
there's a warning; "go tool vet *.go" returns rc 1.
Merge pull request #89 from axw/prealloc-fix-vet

mongo: fix vet warning

The pre-push script's vet invocation, it does nothing.
If you invoke "go tool vet .", it returns rc 0 even if
there's a warning; "go tool vet *.go" returns rc 1.