Avoid ctype.h abuse: cast char to unsigned char first.

[riastradh]

fix #3151

[emanuele6]

Basically the problem is that, if char is signed char on the target system, and the byte that is being checked is non-ascii e.g. 0x80-0xff, it will be sign extended to 0xffffffxx instead of being passed as 0x000000xx, and that is undefined behaviour.

It is wrong to call isspace(x) with a value of x>0x7f anyway because it would be checking a single byte instead of a character, but at least it is not undefined behaviour; let's merge this.

Thank you for the patch.