crypto: handle exceptions in hmac/hash.digest

Forced conversion of the encoding parameter to a string within
crypto.js, fixing segmentation faults in node_crypto.cc.

Fixes: https://github.com/nodejs/node/issues/9819
PR-URL: https://github.com/nodejs/node/pull/12164


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>