fix windows mmap of empty files

src/mmap.cpp

@@ -228,12 +228,12 @@ file_mapping::file_mapping(file_handle file, open_mode_t const mode
 	: m_size(memory_map_size(mode, file_size, file))
 	, m_file(std::move(file), mode, m_size)
 	, m_open_unmap_lock(std::move(open_unmap_lock))
-	, m_mapping((mode & open_mode::no_mmap) ? nullptr
+	, m_mapping((mode & open_mode::no_mmap) || m_size == 0 ? nullptr
 		: MapViewOfFile(m_file.handle(), map_access(mode), 0, 0, static_cast<std::size_t>(m_size)))
 {
 	// you can't create an mmap of size 0, so we just set it to null. We
 	// still need to create the empty file.
-	if (!(mode & open_mode::no_mmap) && m_mapping == nullptr)
+	if (!(mode & open_mode::no_mmap) && m_size > 0 && m_mapping == nullptr)
 		throw_ex<storage_error>(error_code(GetLastError(), system_category()), operation_t::file_mmap);
 }