util/uri: URI member path can be null, compare more carfully

uri_resolve_relative() calls strcmp(bas->path, ref->path).  However,
either argument could be null!  Evidence: the code checks for null
after the comparison.  Spotted by Coverity.

I suspect this was screwed up when we stole the code from libxml2.
There the conditional reads

    xmlStrEqual((xmlChar *)bas->path, (xmlChar *)ref->path)

with

    int
    xmlStrEqual(const xmlChar *str1, const xmlChar *str2) {
	if (str1 == str2) return(1);
	if (str1 == NULL) return(0);
	if (str2 == NULL) return(0);
	do {
	    if (*str1++ != *str2) return(0);
	} while (*str2++);
	return(1);
    }

Fix by replicating libxml2's logic faithfully.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>