crypto: make malloc failure check cross-platform

`malloc(0)` may return NULL on some platforms. Do not report
out-of-memory error unless `malloc` was passed a number greater than
`0`.

PR-URL: https://github.com/nodejs/node/pull/8352


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