Summary:
- check most times after calling snprintf that the buffer didn't fill up. Previously we'd proceed and use `buf_size - len` as the length in subsequent calls, which underflowed as those are unsigned size_t.
- replace some memcpys with snprintf for consistency
Closes https://github.com/facebook/rocksdb/pull/3255

Differential Revision: D6541464

Pulled By: ajkr

fbshipit-source-id: 8610ea6a24f38e0a37c6d17bc65b7c712da6d932