Use unsigned char as argument of std::isspace

Although signature allows an int:

int isspace( int ch );

The behavior is undefined if the value of ch is
not representable as unsigned char and is not
equal to EOF.

See

http://en.cppreference.com/w/cpp/string/byte/isspace
http://www.greenend.org.uk/rjk/tech/cfu.html

This is really a tricky corner case of C standard!

Spotted and reported by Ron Britvich.

No functional change.