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/+/27388Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Alexis Hétu <sugoi@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>