ip route: don't assume default route

Just print the help when "ip route del" is called without any other
arguments.

Resolves:

 * https://bugzilla.redhat.com/show_bug.cgi?id=997965

Signed-off-by: Pavel Šimerda <psimerda@redhat.com>
diff --git a/ip/iproute.c b/ip/iproute.c
index c9cf5d6..32847c6 100644
--- a/ip/iproute.c
+++ b/ip/iproute.c
@@ -975,6 +975,9 @@
 		argc--; argv++;
 	}
 
+	if (!dst_ok)
+		usage();
+
 	if (d || nhs_ok)  {
 		int idx;