Fix clone_constant_impl to correctly deal with null pointers

Summary:
This patch resolves llvm-c-test's following error

```
LLVM ERROR: LLVMGetValueKind returned incorrect type
```

which arises when the input bitcode contains a null pointer.

Reviewers: jdoerfert, CodaFi, deadalnix

Reviewed By: jdoerfert

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68928