fix panic on missing sketch path (#432)

Author: Massimiliano Pippi

Diff for: cli/board/attach.go

@@ -27,6 +27,8 @@ import (
 	"github.com/arduino/arduino-cli/cli/output"
 	"github.com/arduino/arduino-cli/commands/board"
 	rpc "github.com/arduino/arduino-cli/rpc/commands"
+	"github.com/arduino/go-paths-helper"
+	"github.com/sirupsen/logrus"
 	"github.com/spf13/cobra"
 )
 
@@ -52,18 +54,37 @@ var attachFlags struct {
 
 func runAttachCommand(cmd *cobra.Command, args []string) {
 	instance := instance.CreateInstance()
-	var path string
-	if len(args) > 0 {
-		path = args[1]
+
+	var path *paths.Path
+	if len(args) > 1 {
+		path = paths.New(args[1])
+	} else {
+		path = initSketchPath(path)
 	}
+
 	_, err := board.Attach(context.Background(), &rpc.BoardAttachReq{
 		Instance:      instance,
 		BoardUri:      args[0],
-		SketchPath:    path,
+		SketchPath:    path.String(),
 		SearchTimeout: attachFlags.searchTimeout,
 	}, output.TaskProgress())
 	if err != nil {
 		feedback.Errorf("Attach board error: %v", err)
 		os.Exit(errorcodes.ErrGeneric)
 	}
 }
+
+// initSketchPath returns the current working directory
+func initSketchPath(sketchPath *paths.Path) *paths.Path {
+	if sketchPath != nil {
+		return sketchPath
+	}
+
+	wd, err := paths.Getwd()
+	if err != nil {
+		feedback.Errorf("Couldn't get current working directory: %v", err)
+		os.Exit(errorcodes.ErrGeneric)
+	}
+	logrus.Infof("Reading sketch from dir: %s", wd)
+	return wd
+}