simple implementation for isascii where stdlib isascii is not available

[fffaraz]

Some platforms don't have the isascii() function. I had a similar patch for wolfssl as well.
https://github.com/wolfSSL/wolfssl/blob/master/wolfssl/wolfcrypt/types.h#L934

[bradh352]

I'd rather just always use your definition as its always correct. That said, we should cast c to unsigned char when checked so we don't get warnings and we can omit the check for >= 0 since that's always true when unsigned. If you don't do both of those, depending on where its checked, you might get a warning emitted from the compiler about one condition always being true.