Router: fixed crash when matching an empty address pattern array.

A crash would occur when the router tried to match an
against an empty address pattern array.

The following configuration was used to reproduce the
issue:

{
    "listeners": {
        "127.0.0.1:8082": {
            "pass": "routes"
        }
    },
    "routes": [
        {
            "match": {
                "source": []
            },
            "action": {
                "return": 200
            }
        }
    ]
}

src/nxt_http_route.c

@@ -1936,6 +1936,11 @@ nxt_http_route_addr_rule(nxt_http_request_t *r,
     nxt_http_route_addr_pattern_t  *p;
 
     n = addr_rule->items;
+
+    if (n == 0) {
+        return 0;
+    }
+
     p = &addr_rule->addr_pattern[0] - 1;
 
     do {