make gcc shut up

If -flto is used then gcc will generate a lot more warnings than before,
among them a number of use-without-initialization warnings. Most of them
without are false positives, but let's make them go away, because it
doesn't really matter.

src/network/networkd-link.c

@@ -105,7 +105,7 @@ int link_get(Manager *m, int ifindex, Link **ret) {
 }
 
 int link_add(Manager *m, struct udev_device *device, Link **ret) {
-        Link *link;
+        Link *link = NULL;
         Network *network;
         int r;