null pointer exception - decoding "null" type

[vgough]

Using ocf.NewDecoder(r) on a random avro file, the Decode operation panics due to a null pointer exception from hamba/avro. The issue is that newEfaceDecoder inside codec_dynamic does not check the error of genericReceiver and proceeds with a nil typ value, which causes a null pointer exception later in the code.

One fix could be to add a Null condition in genericReceiver in codec_generic.go:

	case Null:
		return reflect2.TypeOf((*null)(nil)), nil

That at least results in a "avro: schema type null in unsupported" error rather than a NPE. Ideally errors should also not be ignored, and turned into some sort of error upward.

[nrwiersma]

Can you provide a small bit a of code that reproduces the issue?