Check if multierror is nil in WrappedErrors

This adds a check similar to `ErrorsOrNil` that ensures the multierror pointer is not nil before returning the Errors field. I believe this is fully backwards-compatible, because the former behavior is a runtime panic.

multierror.go

@@ -40,14 +40,17 @@ func (e *Error) GoString() string {
 	return fmt.Sprintf("*%#v", *e)
 }
 
-// WrappedErrors returns the list of errors that this Error is wrapping.
-// It is an implementation of the errwrap.Wrapper interface so that
-// multierror.Error can be used with that library.
+// WrappedErrors returns the list of errors that this Error is wrapping. It is
+// an implementation of the errwrap.Wrapper interface so that multierror.Error
+// can be used with that library.
 //
-// This method is not safe to be called concurrently and is no different
-// than accessing the Errors field directly. It is implemented only to
-// satisfy the errwrap.Wrapper interface.
+// This method is not safe to be called concurrently. Unlike accessing the
+// Errors field directly, this function also checks if the multierror is nil to
+// prevent a null-pointer panic. It satisfies the errwrap.Wrapper interface.
 func (e *Error) WrappedErrors() []error {
+	if e == nil {
+		return nil
+	}
 	return e.Errors
 }
 

multierror_test.go

@@ -69,6 +69,11 @@ func TestErrorWrappedErrors(t *testing.T) {
 	if !reflect.DeepEqual(multi.Errors, multi.WrappedErrors()) {
 		t.Fatalf("bad: %s", multi.WrappedErrors())
 	}
+
+	multi = nil
+	if err := multi.WrappedErrors(); err != nil {
+		t.Fatalf("bad: %#v", multi)
+	}
 }
 
 func TestErrorUnwrap(t *testing.T) {