Bugfix: initiate_stratum: Ensure extranonce2 size is not negative (which could lead to exploits later as too little memory gets allocated)

Thanks to Mick Ayzenberg <mick@dejavusecurity.com> for finding this!

util.c

@@ -2505,7 +2505,8 @@ resend:
 		goto out;
 	}
 	n2size = json_integer_value(json_array_get(res_val, 2));
-	if (!n2size) {
+	if (n2size < 1)
+	{
 		applog(LOG_INFO, "Failed to get n2size in initiate_stratum");
 		free(sessionid);
 		free(nonce1);