[Memory] Close shared memory FD and properly handle its invalid value

src/xenia/base/memory.h

@@ -18,6 +18,7 @@
 
 #include "xenia/base/assert.h"
 #include "xenia/base/byte_order.h"
+#include "xenia/base/platform.h"
 
 namespace xe {
 namespace memory {
@@ -96,7 +97,15 @@ void AlignedFree(T* ptr) {
 #endif  // XE_COMPILER_MSVC
 }
 
+#if XE_PLATFORM_WIN32
+// HANDLE.
 typedef void* FileMappingHandle;
+constexpr FileMappingHandle kFileMappingHandleInvalid = nullptr;
+#else
+// File descriptor.
+typedef int FileMappingHandle;
+constexpr FileMappingHandle kFileMappingHandleInvalid = -1;
+#endif
 
 FileMappingHandle CreateFileMappingHandle(const std::filesystem::path& path,
                                           size_t length, PageAccess access,