network: Resolve Coverity FORWARD_NULL

Commit id 'ca481a6f' added virNetworkRouteDefFree which may be called
in an error path from lxcAddNetworkRouteDefinition with 'route = NULL'.
So just add the (!def) at the top to resolve.

src/conf/networkcommon_conf.c

@@ -52,6 +52,8 @@ struct _virNetworkRouteDef {
 void
 virNetworkRouteDefFree(virNetworkRouteDefPtr def)
 {
+    if (!def)
+        return;
     VIR_FREE(def->family);
     VIR_FREE(def);
 }