Merge pull request #95 from corani/nil_len

fix: handle nil pointer in Len function of Error struct

sort.go

@@ -4,7 +4,11 @@
 package multierror
 
 // Len implements sort.Interface function for length
-func (err Error) Len() int {
+func (err *Error) Len() int {
+	if err == nil {
+		return 0
+	}
+
 	return len(err.Errors)
 }