Don't panic with "index out of range" on empty string argument.

Fixes golang/go#31277

Change-Id: I005f9523caec76337cb2ec87272a6be4736bce18
Reviewed-on: https://go-review.googlesource.com/c/go/+/170937
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>