netowkrd: initialize variables

Before this, in manager_rtnl_process_rule(), iif and oif are not
initialized. However, message may not contain FRA_IIFNAME or
FRA_IIFNAME. Thus, uninitialized values are set to RoutingPolicyRule
and causes segfault.
This makes iif and oif be initialized as NULL.

Fixes systemd#7704.

src/network/networkd-manager.c

@@ -730,7 +730,7 @@ int manager_rtnl_process_rule(sd_netlink *rtnl, sd_netlink_message *message, voi
         union in_addr_union to, from;
         uint32_t fwmark = 0, table = 0;
         Manager *m = userdata;
-        char *iif, *oif;
+        char *iif = NULL, *oif = NULL;
         uint16_t type;
         int family;
         int r;