An empty label statement can just be ignored, as it cannot
be the target of any gotos.

Tests are already in test/fixedbugs/issue7538*.go

Fixes #11589
Fixes #11593

Change-Id: Iadcd639e7200ce16aa40fd7fa3eaf82522513e82
Reviewed-on: https://go-review.googlesource.com/12093Reviewed-by: Daniel Morsing <daniel.morsing@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>