Summary:
As explained in the comments, Sometimes we create Slice(nullptr, 0) in our code base which cause us to do calls like
```
memcmp(nullptr, "abc", 0);
```
That's fine since the len is equal 0, but UBSAN is not happy about it
so disable UBSAN for this function and add an assert instead
Closes https://github.com/facebook/rocksdb/pull/2616
Differential Revision: D5458326
Pulled By: IslamAbdelRahman
fbshipit-source-id: cfca32abe30f7d8f760c9f77ecd9543dfb1170dd
a4c42e8007