If the requested coding bit sizes don't result in a full binary tree,
then reject the input as invalid.

Exception: We still need to allow degenerate Huffman codings with a
single 1-bit code to be compatible with zlib and files compressed with
Go's compress/flate package.

Update #10426.

Change-Id: I171b98d12e65b4deb9f4031cd802407ebb5e266c
Reviewed-on: https://go-review.googlesource.com/8922Reviewed-by: Nigel Tao <nigeltao@golang.org>