Fix UB when calling cctype functions

[glebm]

Calling cctype functions, such as isspace, with negative values is undefined behaviour.

While glibc allows it, it crashes on uClibc compiled without UCLIBC_HAS_CTYPE_SIGNED.

Fixes the undefined behaviour by casting all arguments to cctype functions to unsigned char.

[glebm]

@Konstanty Any chance you can merge this please? It's been tested by the OpenDingux community and it does resolve a few playback issues. Thanks

[AliceLR]

This looks OK, though I think this library still tries to target C++98. I would recommend C-style casts instead. (@Konstanty @sezero please correct me if I am wrong.)

[glebm]

static_cast is in C++98 https://en.cppreference.com/w/cpp/language/static_cast

[AliceLR]

You're right, nevermind!