gzip: fix compression level bug. Closes 9131

gzip: fix compression level bug. Closes 9131

fix broken logic to get the gzip_level_config value from options -1 to
-9.

This fixes an off-by-one bug that caused gzip -9 output bigger files
than the other compression levels.

It fixes so that compression level 1 to 3 are actually mapped to level 4
as comments say.

It also fixes that levels -4 to -9 is mapped to correct level and avoids
out-of-bounds access.

Signed-off-by: Natanael Copa <ncopa@alpinelinux.org>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>