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

lib/url.c

@@ -3627,7 +3627,7 @@ static void fix_hostname(struct SessionHandle *data,
   host->dispname = host->name;
 
   len = strlen(host->name);
-  if(host->name[len-1] == '.')
+  if(len && (host->name[len-1] == '.'))
     /* strip off a single trailing dot if present, primarily for SNI but
        there's no use for it */
     host->name[len-1]=0;