crypto: do not use pointers to std::vector

The pointer to std::vector is unnecessary, so replace it with standard
instance. Also, make the for() loop more readable by using actual type
instead of inferred - there is no readability benefit here from
obfuscating the type.

PR-URL: https://github.com/nodejs/node/pull/8334


Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>