Fix stack initialization when running under .NET.

Under .NET, on WinXP and sometimes on Win7 we will have two blocks
of uncommited memory before the guard page, so look one block further.

In debug mode it would crash due to assert
guardPage.Protect & PAGE_GUARD

In release mode it would crash due to only using a 4K stack.

Change-Id: If9f2c84122ba95a18403a8f413d080dd5cc58250
Reviewed-by: Michael Bruning <michael.bruning@digia.com>