add assert to make sure a failed typed stack pop doesn't remove the item from the stack

Signed-off-by: David Lawrence <david.lawrence@docker.com> (github: endophage)

tuf/utils/stack_test.go

@@ -50,6 +50,7 @@ func TestPopStringWrongType(t *testing.T) {
 	_, err := s.PopString()
 	assert.Error(t, err)
 	assert.IsType(t, ErrBadTypeCast{}, err)
+	assert.Len(t, s.s, 1)
 }
 
 func TestPopStringEmpty(t *testing.T) {