fix out of bounds panic with "garble build"

Forgot to run all the tests with the last commit.

main.go

@@ -151,10 +151,12 @@ func mainErr(args []string) error {
 	case "help":
 		flagSet.Usage()
 	case "build", "test":
+		if len(args) > 1 {
 			switch args[1] {
 			case "-h", "-help", "--help":
 				flagSet.Usage()
 			}
+		}
 		wd, err := os.Getwd()
 		if err != nil {
 			return err