Len() with nil multierror should return 0 instead of panic

[maxiride]

As of now the method can be called only on a not nil Error.

I stumbled upon the situation where I wanted handle how many errors a multierror variable contained, however Len can't be called on a nil multierror (which would cause a runtime panic).

This implies that a nil multierror and zero length multierror are treated differently. I would expect the example in the playground to return 0 instead of throwing a panic. Is this an intentional design choice or maybe there is room for improvement?

https://play.golang.org/p/n7fwsjBybhs

The Len() method could be changed as the following without breaking changes (I think):

func (err *Error) Len() int {
	if err != nil {
		return len(err.Errors)
	}

	return 0
}

Otherwise one would need to go through the ErrOrNil method with little, although not needed, overhead.

         var merr *multierror.Error
	if merr.ErrOrNil != nil {
		fmt.Println(merr.Len())
	} else {
		fmt.Println("0")
	}

---

I should also add that his condition happens when a multierror is initialized but nothing has been appended to it, in fact appending nil errors correctly fills it with zeroes.
https://play.golang.org/p/B3P_aNnms2W