Don't panic, crash, or return references to uninitialized memory when
ParseUnixCredentials is passed invalid input.

Fixes #16475

Change-Id: I140d41612e8cd8caaa94be829a415159659c217b
Reviewed-on: https://go-review.googlesource.com/25154
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>