gzip: fix GCC warnings about signedness
gzip: fix GCC warnings about signedness

This resolves the following warning occurring on GCC.

> warning: pointer targets in assignment from ‘char *’ to
> ‘uint8_t *’ {aka ‘unsigned char *’} differ in signedness

Since the buffer pointer was declared as (uint8_t *), I should have
casted the value to unsigned char, not signed one. Fix it.

Signed-off-by: Fujimoto Seiji <ef879192bb1e41d519c82cf76440dc8e9a530806@clear-code.com>

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