http2/hpack: fix nil pointer dereference crash in huffman decoder

Return an error instead.

Fixes bradfitz/http2#56

Change-Id: I3d1e80a214a8635932479943f0ef9610ee02b233
Reviewed-on: https://go-review.googlesource.com/15738
Reviewed-by: Andrew Gerrand <adg@golang.org>
4 files changed