Original code is mistakenly panics on VirtualAlloc failure - we want
it to go looking for smaller memory region that VirtualAlloc will
succeed to allocate. Also return immediately if VirtualAlloc succeeds.
See rsc comment on issue #12587 for details.

I still don't have a test for this. So I can only hope that this

Fixes #12587

Change-Id: I052068ec627fdcb466c94ae997ad112016f734b7
Reviewed-on: https://go-review.googlesource.com/17169Reviewed-by: Russ Cox <rsc@golang.org>