runtime: fix handling VirtualAlloc failure in sysUsed

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>

src/runtime/mem_windows.go

@@ -58,9 +58,8 @@ func sysUnused(v unsafe.Pointer, n uintptr) {
 
 func sysUsed(v unsafe.Pointer, n uintptr) {
 	r := stdcall4(_VirtualAlloc, uintptr(v), n, _MEM_COMMIT, _PAGE_READWRITE)
-	if r != uintptr(v) {
-		print("runtime: VirtualAlloc of ", n, " bytes failed with errno=", getlasterror(), "\n")
-		throw("runtime: failed to commit pages")
+	if r == uintptr(v) {
+		return
 	}
 
 	// Commit failed. See SysUnused.