Avoid Metal validation error on MTLBuffer.contents access from private storage.
diff --git a/MoltenVK/MoltenVK/GPUObjects/MVKDeviceMemory.mm b/MoltenVK/MoltenVK/GPUObjects/MVKDeviceMemory.mm
index 0e3c194..f1cbbeb 100644
--- a/MoltenVK/MoltenVK/GPUObjects/MVKDeviceMemory.mm
+++ b/MoltenVK/MoltenVK/GPUObjects/MVKDeviceMemory.mm
@@ -149,7 +149,7 @@
 	} else {
 		_mtlBuffer = [getMTLDevice() newBufferWithLength: memLen options: _mtlResourceOptions];     // retained
 	}
-	_pMemory = _mtlBuffer.contents;
+	_pMemory = isMemoryHostAccessible() ? _mtlBuffer.contents : nullptr;
 
 	return true;
 }