Fix buffer overrun problem (and allow to decode unpadded data)

As per RFC 2045, data to be decoded does not need to be padded with equal signs to make the size of the input data a multiple of 4 bytes. Such data caused a buffer overrun in releases prior to TV2.rc.08.

V2, release candiate 7

Merged improvements (mostly by Pablo Martin-Gomez)

V2, release candidate 4

Fixing implicit cast warnings (to/from size_t).

V2, release candidate 3

Fix concurrency issue in mutlithreaded environment (2.rc.03)

Problem:
If two threads called base64_encode concurrently (one with url=true and
the other with url=false, the values of base64_chars[62] and
base64_chars[63] are undefined. For example, it's possible to get
base64_chars[62]='-' and base64_chars[63]='/', which is not a valid
encoding.

This commit fixes this issue.