Base64 should not rely on std::isalnum

[jpcima]

According to the man page of isalnum:

In some locales, there may be additional characters for which isalpha() is true—letters which are neither uppercase nor lowercase.

This means that the function might let through characters outside the base64 alphabet.

reference:

DPF/distrho/extra/Base64.hpp

Line 81 in 86a621b

return (std::isalnum(c) || (c == '+') || (c == '/'));