Use default_interface_conf in add_interface, not in main.

This avoids a crash when an interface is added both in the configuration
file and on the command-line.

babeld.c

@@ -368,7 +368,7 @@ main(int argc, char **argv)
     }
 
     for(i = optind; i < argc; i++) {
-        vrc = add_interface(argv[i], default_interface_conf);
+        vrc = add_interface(argv[i], NULL);
         if(vrc == NULL)
             goto fail;
     }

interface.c

@@ -75,7 +75,7 @@ add_interface(char *ifname, struct interface_conf *if_conf)
 
     memset(ifp, 0, sizeof(struct interface));
     strncpy(ifp->name, ifname, IF_NAMESIZE);
-    ifp->conf = if_conf;
+    ifp->conf = if_conf ? if_conf : default_interface_conf;
     ifp->bucket_time = now.tv_sec;
     ifp->bucket = BUCKET_TOKENS_MAX;
     ifp->hello_seqno = (random() & 0xFFFF);