[release-branch.go1.5] runtime: avoid stat underflow crash

If the area returned by sysReserve in mheap.sysAlloc is outside the
usable arena, we sysFree it. We pass a fake stat pointer to sysFree
because we haven't added the allocation to any stat at that point.
However, we pass a 0 stat, so sysFree panics when it decrements the
stat because the fake stat underflows.

Fix this by setting the fake stat to the allocation size.

Updates #13143 (this is a prerequisite to fixing that bug).

Change-Id: I61a6c9be19ac1c95863cf6a8435e19790c8bfc9a
Reviewed-on: https://go-review.googlesource.com/16926


Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-on: https://go-review.googlesource.com/16987


Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>