Fixes the following security issue:
- CVE-2019-18408: archive_read_format_rar_read_data in
archive_read_support_format_rar.c in libarchive before 3.4.0 has a
use-after-free in a certain ARCHIVE_FAILED situation, related to
Ppmd7_DecodeSymbol.
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>