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>

--- a/archival/gzip.c
+++ b/archival/gzip.c
@@ -2220,10 +2220,7 @@ int gzip_main(int argc UNUSED_PARAM, cha
 	opt >>= ENABLE_GUNZIP ? 7 : 5; /* drop cfv[dt]qn bits */
 	if (opt == 0)
 		opt = 1 << 6; /* default: 6 */
-	/* Map 1..3 to 4 */
-	if (opt & 0x7)
-		opt |= 1 << 4;
-	opt = ffs(opt >> 3);
+	opt = ffs(opt >> 4); /* Maps -1..-4 to [0], -5 to [1] ... -9 to [5] */
 	max_chain_length = 1 << gzip_level_config[opt].chain_shift;
 	good_match	 = gzip_level_config[opt].good;
 	max_lazy_match	 = gzip_level_config[opt].lazy2 * 2;