gunzip: code shrink by using int-, not short-sized struct member

gunzip: code shrink by using int-, not short-sized struct member

function                                             old     new   delta
inflate_block                                       1254    1253      -1
inflate_codes                                        629     621      -8
huft_build                                          1054    1008     -46
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/3 up/down: 0/-55)             Total: -55 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>