uncompress: Avoid reading an extra byte

When reading the next code in a stream, avoid reading an extra byte if
we're going to throw it away. When there's no more bits to extract from
the stream, bits will be 0 and we'll mask the read byte with 0 anyway.
At worst, this will avoid reading one past the end of gbuf array (which
is not possible in well formed streams).

PR: 127912
Reviewed by:	emaste
Differential Revision:	https://reviews.freebsd.org/D47041

usr.bin/compress/zopen.c

@@ -620,7 +620,8 @@ getcode(struct s_zstate *zs)
 	}
 
 	/* High order bits. */
-	gcode |= (*bp & rmask[bits]) << r_off;
+	if (bits > 0)
+		gcode |= (*bp & rmask[bits]) << r_off;
 	roffset += n_bits;
 
 	return (gcode);