meta: fix issue with errors.WithStack, io.Copy expect io.EOF

We cannot use errors.WithStack [1] to maintain call stack information
of zeros.Read as this method is invoked from io.Copy (in verifyPadding)
which expects io.EOF.

Note, the type of the returned error in the case of EOF becomes,
"errors.withStack" with the concrete value:
	errors.withStack{error: io.EOF, stack: callers()}

This broke the tests:

https://travis-ci.org/mewkiz/flac/builds/383805346#L540

We cannot update the standard library to check for io.EOF
using errors.Cause(err) == io.EOF. Thus, this seems like the
only solution.

Unfortunate, and a bit scare, as there may be other use of
interfaces which break since the error returned is wrapped
with added context.

Hope this gets resolved in Go 2. I know @bradfitz among others
have raised their interest in finding ways to improve and further
simplify error handling mechanisms [2].

[1]: https://github.com/pkg/errors
[2]: https://github.com/golang/go/wiki/Go2

Updates #22.

Related to golang/go#21161.

meta/padding.go

@@ -36,5 +36,7 @@ func (zr zeros) Read(p []byte) (n int, err error) {
 			return n, ErrInvalidPadding
 		}
 	}
-	return n, errors.WithStack(err)
+	// Note, not wrapping err in errors.WithStack as it will confuse io.Copy
+	// which is expecting io.EOF.
+	return n, err
 }