nil check when wrapping errors fails for custom error types

[jeanspector-google]

nil check when wrapping errors fails for custom error types. nil error get wrapped and - with pointers - it even results in a panic.

Here's a UT that demonstrates the behavior:

package go_err

import (
	"testing"

	"github.com/go-errors/errors"
)

type MyError1 struct {
	message string
}

func (me1 MyError1) Error() string{
	return me1.message
}

type MyError2 struct {
	message string
}

func (me2 *MyError2) Error() string{
	return me2.message
}

func TestNilErr(t *testing.T) {
	var (
		err error
		myErr1 MyError1
		myErr2 *MyError2
	)

	err = errors.WrapPrefix(err, "blah message", 0)
	t.Log("base error", "value", err)

	// Shouldn't be wrapped, but it is
	err = errors.WrapPrefix(myErr1, "blah message", 0)
	t.Log("my error 1", "value", err)

	// Shouldn't be wrapped, but it panics
	err = errors.WrapPrefix(myErr2, "blah message", 0)
	t.Log("my error 2", "value", err)
}

[jeanspector-google]

Sent pull request #36 that should fix the issue