deps: update comment about PURIFY define

PURIFY makes OpenSSL zero out some buffers.  It also stops RAND_bytes()
from using the existing contents of the destination buffer as a source
of entropy, which according to some papers, is a possible attack vector
for reducing the overall entropy.

PR-URL: https://github.com/nodejs/node/pull/6582


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>