criu: Require @fd argument for "swrk" mode

Otherwise we hit nil dereference.
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

crtools.c

@@ -269,6 +269,8 @@ int main(int argc, char *argv[], char *envp[])
 		return 1;
 
 	if (!strcmp(argv[1], "swrk")) {
+		if (argc < 3)
+			goto usage;
 		/*
 		 * This is to start criu service worker from libcriu calls.
 		 * The usage is "criu swrk <fd>" and is not for CLI/scripts.