added upc length limiters and fixed 10 digit bug

main.go

@@ -48,6 +48,19 @@ func main() {
 		evenNumber := 0
 		var remainder int
 
+		if upcLength < 10 {
+			gtins = append(gtins, "Cannot process upcs less than 10 digits")
+			continue
+		}
+
+		if upcLength == 10 {
+			upc = "0" + upc
+		}
+
+		if upcLength > 13 {
+			gtins = append(gtins, "GTINs longer than 14 digits are not valid")
+		}
+
 		for i, num := range upc {
 			if i%2 == 0 {
 				oddString, err := strconv.Atoi(string(num))