BUG/MINOR: http: fix an out-of-bounds read in http_get_host_port() on empty host

http_get_host_port() walks backwards from the end of the host looking for the
first non-digit, then checks whether it is a colon:

	start = istptr(host);
	end = istend(host);
	for (ptr = end; ptr > start && isdigit((unsigned char)*--ptr););

	/* no port found */
	if (likely(*ptr != ':'))
		return IST_NULL;

When <host> is empty, the loop condition fails immediately, the pre-decrement
is never evaluated and <ptr> is left equal to <end>, so *ptr reads one byte
past the end of the string. With an IST_NULL argument this is a NULL
dereference.

Both cases are reachable with an empty host: h1_validate_mismatch_authority()
calls it on the Host header value, which may be empty ("Host:\r\n") while an
absolute-form request URI is used, and http_scheme_based_normalize() calls it
on the authority extracted from the URI, which is empty for a request like
"GET http:///x HTTP/1.1". In practice the extra byte always lies inside the
request buffer, so the observable effect is limited to possibly mistaking a
neighbour byte for a colon and returning a bogus port, but it remains an
out-of-bounds read and the helper must be usable with an unset ist.

Let's return IST_NULL right away for an empty host.

This was introduced by commit 658f97162 ("MINOR: http: Add function to get port
part of a host") in 2.7-dev2, so it should be backported to 2.8 and above.