Safer alloca?

gistfile1.txt

void* GetStackBase()
{
    static __declspec(thread) void* stackBase = nullptr;
    if (stackBase == nullptr)
    {
        MEMORY_BASIC_INFORMATION mbi;
        VirtualQuery(&mbi, &mbi, sizeof(mbi));
        stackBase = mbi.AllocationBase;
    }
    return stackBase;
}

bool GetIsRoomOnStack(size_t size)
{
    char dummy;
    return &dummy - size > GetStackBase();
}

#define Alloca(size) \
    (GetIsRoomOnStack((size)) ? alloca((size)) : malloc((size)))