Fix undefined behaviour and sign extension issues in kstrtok

Fix undefined behaviour and sign extension issues in kstrtok

Prevent read beyond the end of sep if it's an empty string.

Change to internally use unsigned char to avoid unwanted sign
extension on platforms where 'char' is signed.

Fix undefined behaviour where pointers were made to the element
before the start of `str`:
https://www.securecoding.cert.org/confluence/display/c/ARR30-C.+Do+not+form+or+use+out-of-bounds+pointers+or+array+subscripts