Bugfix - Handle invalid characters when decoding

[Rican7]

When using this library in a project, I ran into an issue when I wrote a test that passed intentionally "bad data" to the decode method.

Specifically, when passing the invalid data to the decode method of the GmpEncoder, which uses gmp_init(), the GMP extension was throwing an unchecked warning about the data not being a valid number within the specified base (62).

I've recreated this error in the provided tests, along with a fix:

... Unfortunately, there's not a great way to check for this error at a higher level, due to GMP's usage of warnings and a false return value. Interestingly enough, I've actually run into this problem before when building my own library years ago, so I've actually reported this as a PHP bug, but it was closed with a "Won't Fix" status: https://bugs.php.net/bug.php?id=68002

In any case, I've created this fix to handle the issue for both the GMP encoder's case and for the base encoder, as otherwise not handling this case could lead to an invalid decode result or "data-loss".

[tuupola]

Thanks! Today I also learned about strspn() :)

[Rican7]

Ha, thank YOU for the merge!

I hadn't known about strspn myself either, but I went and dug through the various PHP string methods, because I just figured that there was something in there that could do it... I just didn't expect it to work so well with a string character set, haha. But hell, if there's anything I've learned about PHP in the 13+ years that I've been writing it, its that the standard library has amazing string utilities... as long as you don't care about UTF-8. 😆

In any case, thanks for the merge and for the open source project! I'll be looking forward to a release. 😃