vanitygen: fix integer overflow

cmd/chantools/vanitygen.go

@@ -73,6 +73,10 @@ func (c *vanityGenCommand) Execute(_ *cobra.Command, _ []string) error {
 	if len(prefixBytes) < 2 {
 		return fmt.Errorf("prefix must be at least 2 bytes")
 	}
+	if len(prefixBytes) > 8 {
+		return fmt.Errorf("prefix too long, unlikely to find a key " +
+			"within billions of years")
+	}
 	if !(prefixBytes[0] == 0x02 || prefixBytes[0] == 0x03) {
 		return fmt.Errorf("prefix must start with 02 or 03 because " +
 			"it's an EC public key")