Unlike RFC 1951 (DEFLATE), bzip2 does not use zero-length Huffman codes
to indicate that the symbol is missing. Instead, bzip2 uses a sparse
bitmap to indicate which symbols are present. Thus, it is undefined what
happens when a length of zero is used. Thus, fix the parsing logic so that
the length cannot ever go below 1-bit similar to how the C logic does things.

To confirm that the C bzip2 utility chokes on this data:
	$ echo "425a6836314159265359b1f7404b000000400040002000217d184682ee48
	a70a12163ee80960" | xxd -r -p | bzip2 -d

	bzip2: Data integrity error when decompressing

For reference see:
	bzip2-1.0.6/decompress.c:320

Change-Id: Ic1568f8e7f80cdea51d887b4d712cc239c2fe85e
Reviewed-on: https://go-review.googlesource.com/20119
Run-TryBot: Joe Tsai <joetsai@digital-static.net>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>