Allow null dispatchable objects

Because we need to dereference dispatchable objects in order to get
the underlying object, we need to first check for VK_NULL_HANDLE to
avoid crashing.

Bug b/116336664

Change-Id: Ia903dcc6142d5e7f283da8fbef0f43bae3ca243f
Tests: dEQP-VK.api.null_handle.free_command_buffers
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/27388
Reviewed-by: Nicolas Capens <[email protected]>
Tested-by: Alexis Hétu <[email protected]>
Kokoro-Presubmit: kokoro <[email protected]>

Author: sugoi1

src/Vulkan/VkObject.hpp

@@ -144,7 +144,8 @@ class DispatchableObject
 
 	static inline T* Cast(VkT vkObject)
 	{
-		return &(reinterpret_cast<DispatchableObject<T, VkT>*>(vkObject)->object);
+		return (vkObject == VK_NULL_HANDLE) ? nullptr :
+		       &(reinterpret_cast<DispatchableObject<T, VkT>*>(vkObject)->object);
 	}
 
 	operator VkT()