busybox: Fix for CVE-2021-42374

An out-of-bounds heap read in unlzma leads to information leak and
denial of service when crafted LZMA-compressed input is decompressed.
This can be triggered by any applet/format that internally supports
LZMA compression.

Reference: https://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2021-42374



Signed-off-by: Pavel Zhukov <pavel.zhukov@huawei.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>