suggested change to avoid msvc iterator check assertions in uri parser

this is a suggestion for a fix to avoid triggering iterator check assertions when parsing "non complete" uris on windows in debug mode with "checked iterators" enabled.

the basic issue is that in case of a missing element (e.g. host) an empty iterator_range with default-constructed iterators is inserted. default iterators are initialized to "singular" values which trigger microsofts iterator checks.

P.S. please disregard commits other than db9b2bb, i'm a bit of a git newbie...