Summary:
- removed a few unneeded variables
- fused some variable declarations and their assignments
- fixed right-trimming code in string_util.cc to not underflow
- simplifed an assertion
- move non-nullptr check assertion before dereferencing of that pointer
- pass an std::string function parameter by const reference instead of by value (avoiding potential copy)
Closes https://github.com/facebook/rocksdb/pull/3507

Differential Revision: D7004679

Pulled By: sagar0

fbshipit-source-id: 52944952d9b56dfcac3bea3cd7878e315bb563c4