fix panic for invalid task in cyclic dep detection

resolves partly #37

cyclic.go

@@ -13,7 +13,12 @@ func (e *Executor) CheckCyclicDep() error {
 		defer delete(visits, name)
 
 		for _, d := range t.Deps {
-			if err := checkCyclicDep(d.Task, e.Tasks[d.Task]); err != nil {
+			// FIXME: ignoring by now. should return an error instead?
+			task, ok := e.Tasks[d.Task]
+			if !ok {
+				continue
+			}
+			if err := checkCyclicDep(d.Task, task); err != nil {
 				return err
 			}
 		}

cyclic_test.go

@@ -35,4 +35,22 @@ func TestCyclicDepCheck(t *testing.T) {
 	}
 
 	assert.NoError(t, isNotCyclic.CheckCyclicDep())
+
+	inexixtentTask := &task.Executor{
+		Tasks: task.Tasks{
+			"task-a": &task.Task{
+				Deps: []*task.Dep{&task.Dep{Task: "invalid-task"}},
+			},
+		},
+	}
+
+	// FIXME: by now Task should ignore non existent tasks
+	// in the future we should improve the detection of
+	// tasks called with interpolation?
+	//     task:
+	//       deps:
+	//         - task: "task{{.VARIABLE}}"
+	//       vars:
+	//         VARIABLE: something
+	assert.NoError(t, inexixtentTask.CheckCyclicDep())
 }