util: Prevent libvirtd crash from virNetDevOpenvswitchAddPort()

* src/util/virnetdevopenvswitch.c (virNetDevOpenvswitchAddPort): avoid libvirtd
crash due to derefing a NULL virtVlan->tag.

RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=852383Signed-off-by: NAlex Jia <ajia@redhat.com>

src/util/virnetdevopenvswitch.c

@@ -99,7 +99,7 @@ int virNetDevOpenvswitchAddPort(const char *brname, const char *ifname,
                 virBufferAddLit(buf, ",");
                 virBufferAsprintf(buf, "%d", virtVlan->tag[i]);
             }
-        } else {
+        } else if (virtVlan->nTags) {
             virBufferAsprintf(buf, "tag=%d", virtVlan->tag[0]);
         }
     }