Fix error handling of if_nametoindex.

babel.c

@@ -273,8 +273,8 @@ main(int argc, char **argv)
         int mtu;
 
         ifindex = if_nametoindex(*arg);
-        if(ifindex < 0) {
-            perror("if_nametoindex");
+        if(ifindex <= 0) {
+            fprintf(stderr, "Unknown interface %s.\n", *arg);
             goto fail;
         }
 

kernel.c

@@ -212,7 +212,7 @@ kernel_route(int add, const unsigned char *dest, unsigned short plen,
 
     if(ifindex_lo < 0) {
         ifindex_lo = if_nametoindex("lo");
-        if(ifindex_lo < 0)
+        if(ifindex_lo <= 0)
             return -1;
     }