BUG/MINOR: tools: url2sa reads too far when no port nor path

BUG/MINOR: tools: url2sa reads too far when no port nor path

url2sa() still have an unfortunate case where it reads 1 byte too far,
it happens when no port or path are specified in the URL, and could
crash if the byte after the URL is not allocated (mostly with ASAN).

This case is never triggered in old versions of haproxy because url2sa
is used with buffers which are way bigger than the URL. It is only
triggered with the httpclient.

Should be bacported in every stable branches.