Return if ssleay_rand_add called with zero num.

Treat a zero length passed to ssleay_rand_add a no op: the existing logic
zeroes the md value which is very bad. OpenSSL itself never does this
internally and the actual call doesn't make sense as it would be passing
zero bytes of entropy.

Thanks to Marcus Meissner <meissner@suse.de> for reporting this bug.
(cherry picked from commit 5be1ae28)