Calling `authority` on URI with empty-string host crashes

[Craig M. Brandenburg]

This issue is similar to Issue #62, including the root cause. See that issue for more details.

For this issue, the following program exposes the crash.

auto uri = network::uri_builder()
    .authority(":1234")
    .uri();
std::cout << *uri.authority() << '\n';

The problem is the same as with #62: calling uri.authority causes a read access violation.