registry/session: fix panic in GetRemoteImageLayer

Fixes #7860
@unclejack I think the documentation is just unclear on that and I hope they'll clarify: 
https://code.google.com/p/go/issues/detail?id=8633&thanks=8633&ts=1409757726

But I'm pretty sure we don't need to close body. If you look at the examples here: https://godoc.org/net/http They never close body in the error handling.