mrt.DecodeSingle does not handle EOF's gracefully

[benjojo]

When DecodeSingle encounters a EOF from the reader, it will report Decoding of type 0 not implemented

It would be nicer if it could pass down the EOF rather than giving a cryptic error, Seems as simple as testing for a EOF here: https://github.com/cloudflare/fgbgp/blob/master/mrt/mrt.go#L790 and then passing up the EOF object.