The code in mem_bsd.go expects that when mmap fails it will return a
positive errno value.  This fixes the Solaris implementation of mmap to
work as expected.

Change-Id: Id1c34a9b916e8dc955ced90ea2f4af8321d92265
Reviewed-on: https://go-review.googlesource.com/19477
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>