util: permit querying a VF MAC address or VLAN tag by itself

virNetDevParseVfConfig() assumed that both the MAC address and VLAN
tag pointers were valid, so even if you only wanted one or the other,
you would need a variable to hold the returned value for both. This
patch checks each for a NULL pointer before filling it in.

src/util/virnetdev.c

@@ -1603,7 +1603,7 @@ virNetDevParseVfConfig(struct nlattr **tb, int32_t vf, virMacAddrPtr mac,
             goto cleanup;
         }
 
-        if (tb[IFLA_VF_MAC]) {
+        if (mac && tb[IFLA_VF_MAC]) {
             vf_mac = RTA_DATA(tb_vf[IFLA_VF_MAC]);
             if (vf_mac && vf_mac->vf == vf)  {
                 virMacAddrSetRaw(mac, vf_mac->mac);
@@ -1611,7 +1611,7 @@ virNetDevParseVfConfig(struct nlattr **tb, int32_t vf, virMacAddrPtr mac,
             }
         }
 
-        if (tb[IFLA_VF_VLAN]) {
+        if (vlanid && tb[IFLA_VF_VLAN]) {
             vf_vlan = RTA_DATA(tb_vf[IFLA_VF_VLAN]);
             if (vf_vlan && vf_vlan->vf == vf)  {
                 *vlanid = vf_vlan->vlan;