BUG/MINOR: tools: fix url2sa return value with IPv4

BUG/MINOR: tools: fix url2sa return value with IPv4

Fix 8a91374 ("BUG/MINOR: tools: url2sa reads ipv4 too far") introduced a
regression in the value returned when parsing an ipv4 host.

Tthe consumed length is supposed to be as far as the first character of
the path, only its not computed correctly anymore and return the length
minus the size of the scheme.

Fixed the issue by reverting 'curr' and 'url' as they were before the
patch.

Must be backported in every stable branch where the 8a91374 patch was
backported.