urlapi: fix parsing ipv6 with zone index

The previous fix for parsing IPv6 URLs with a zone index was a paddle
short for URLs without an explicit port. This patch fixes that case
and adds a unit test case.

This bug was highlighted by issue #3408, and while it's not the full
fix for the problem there it is an isolated bug that should be fixed
regardless.

Closes #3411
Reported-by: GitYuanQu on github
Reviewed-by: Daniel Stenberg <daniel@haxx.se>