[boost] algorithm/string/iterator_range: copy-construction from singular iterator

Hi, Almost all is said in the subject. According to 24.1-5 it is undefined behavior; consequently default initialized parameters cannot be used. So here comes a patch. I didn't put the half constructor (only a "begin" parameter, no "end" parameter) because I don't think it makes sense; but if it does, just add it. Regards, Guillaume