[BUG] we could segfault during exit while freeing uri_auths

The following config makes haproxy segfault on exit :

defaults
	mode	http
	balance	roundrobin

listen  no-stats
        bind       :8001

listen  stats
        bind       :8002
	stats      uri /stats

The simple fix is to ensure that p->uri_auth is not NULL
before dereferencing it.

src/haproxy.c

@@ -737,7 +737,7 @@ void deinit(void)
 				if (uap == p->uri_auth)
 					break;
 
-			if (!uap) {
+			if (!uap && p->uri_auth) {
 				/* add it, if it is */
 				p->uri_auth->next = ua;
 				ua = p->uri_auth;