mod_deflate: read gzip extra field length as unsigned char

[arshsmith1]

consume_zlib_flags() reads the two gzip extra-field (FEXTRA) length bytes through a plain char, so on platforms where char is signed a length byte with the high bit set gets sign-extended and consume_len ends up near UINT_MAX instead of the real 0-65535 length. That makes the DEFLATE input filter (inflating a client gzip request body) and the INFLATE output filter (e.g. a proxied gzip response) mis-parse the header of any gzip stream whose extra-field length low byte is >= 0x80; an ordinary 200-byte extra field (XLEN 0x00C8) already trips it, after which the remainder of the member is swallowed as if it were still the extra field. Reading each byte through unsigned char before widening keeps the decode correct, with no change for any stream whose low length byte is under 0x80.

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