os used to panic (access out of bounds) if executablePath was left empty

Fixes #22529

Change-Id: Iead5e60a3b000dbde421a8e8612c3690340879ce
Reviewed-on: https://go-review.googlesource.com/127546
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>