This code's test coverage is ad hoc at best, and it's easy to make
changes that accidentally regress invariants.  This CL adds a "sanity"
constant that can be changed to "true" during development to add extra
runtime checking that the Huffman decoder tables are sane.

Change-Id: I0d0ca53ad7c9566be18046d9b255e1a30059f28b
Reviewed-on: https://go-review.googlesource.com/8974Reviewed-by: Nigel Tao <nigeltao@golang.org>