curl: URL API: triple-slash parses path segment as hostname

Summary libcurl's URL API misparses http:///host/path. Per RFC 3986, the authority between // and / is empty, so /host/path is the path. But parseurl in lib/urlapi.c treats the first segment as the hostname. On line 1087, the slash counter i reaches 3, which is within the valid 1-3 range, and hos...