As per iant suggestion during issue #12587 crash investigation.

Also adjust incorrect throw message in sysUsed while we are here.

Change-Id: Ice07904fdd6e0980308cb445965a696d26a1b92e
Reviewed-on: https://go-review.googlesource.com/14633Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>