fix_hostname: zero length host name caused -1 index offset

If a URL is given with a zero-length host name, like in "http://:80" or
just ":80", `fix_hostname()` will index the host name pointer with a -1
offset (as it blindly assumes a non-zero length) and both read and
assign that address.

CVE-2015-3144

Bug: http://curl.haxx.se/docs/adv_20150422D.html
Reported-by: Hanno Böck