The status buffer built by the exit function
was not nil-terminated.

Fixes #10789.

Change-Id: I2d34ac50a19d138176c4b47393497ba7070d5b61
Reviewed-on: https://go-review.googlesource.com/9953Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>