Btrfs-progs: fix arguments check of qgroup limit

Segmentation fault occurred in the following command.

 # btrfs qgroup limit
 Segmentation fault

So, we should check a minimum number of arguments.

Signed-off-by: Tsutomu Itoh <t-itoh@jp.fujitsu.com>

cmds-qgroup.c

@@ -374,6 +374,9 @@ static int cmd_qgroup_limit(int argc, char **argv)
 		}
 	}
 
+	if (check_argc_min(argc - optind, 2))
+		usage(cmd_qgroup_limit_usage);
+
 	if (!parse_limit(argv[optind], &size)) {
 		fprintf(stderr, "Invalid size argument given\n");
 		return 1;