Handle NULL gracefully in json_tokener_free

Similarly to glibc's free, make json_tokener_free(NULL)
a no-op, to simplify cleanup paths.

Signed-off-by: Ján Tomko <jtomko@redhat.com>

json_tokener.c

@@ -182,6 +182,8 @@ struct json_tokener *json_tokener_new(void)
 
 void json_tokener_free(struct json_tokener *tok)
 {
+	if (!tok)
+		return;
 	json_tokener_reset(tok);
 	if (tok->pb)
 		printbuf_free(tok->pb);