tc/tc_filter: Make sure filter name is not empty

The later check for 'k[0] != 0' requires a non-empty filter name,
otherwise NULL pointer dereference in 'q' might happen.

Signed-off-by: Phil Sutter <phil@nwl.cc>

tc/tc_filter.c

@@ -412,6 +412,9 @@ static int tc_filter_get(int cmd, unsigned int flags, int argc, char **argv)
 			usage();
 			return 0;
 		} else {
+			if (!**argv)
+				invarg("invalid filter name", *argv);
+
 			strncpy(k, *argv, sizeof(k)-1);
 
 			q = get_filter_kind(k);