Fixed empty string crash

[vsergeenko777]

This code will crash the server:

new fmt_str[128 + 1];
hex_encode("", fmt_str); // crash
sha256("", fmt_str); // crash
whirlpool("", fmt_str); // crash

However, hash functions can return valid values for empty strings. For example, MD5: d41d8cd98f00b204e9800998ecf8427e.

[Mellnik]

Thank you for your pull request. I have incorporated both of you changes into 0.0.5 (dev branch).
Everyone feel free to test it out: https://github.com/Mellnik/hash-plugin/releases/tag/0.0.5