A basic base62 encoder

Inspiried by Base62.java by John Jiyang Hou posted at
https://www.codeproject.com/articles/1076295/base-encode

I've made an attempt to make it somewhat similar in spirit, however
this is C and the idoms are different than java thus I elected to
allow for some variation especially with regards to boundary
conditions around buffer sizes and the error handling there-of.

With luck, data encoding and decoding should be the same between
John's Java version and this C version.