Switched to using zero-valued bytes.Reader.
Switched to using zero-valued bytes.Reader.

This commit switches to using the new zero-value bytes.Reader support
added in Go 1.12.

Signed-off-by: Jacob Howard <f862f167b85d41b225785c70d70808bc7337c1fe@havoc.io>

havoc-io/mutagen,havoc-io/mutagen