4 bits are read ([0, 15]) with 4 added to the result. The check that the
result is not > NUM_CODE_LENGTH_CODES (19) is redundant. This makes the
check an assert for documentation purposes.
webp-lossless-bitstream-spec.txt is updated accordingly.
Bug: webp:611
Change-Id: Ie461017dc9de3da4cfe6856543d3dcf58f53a180