XZ decompressor: Fix decoding of empty LZMA2 streams

XZ decompressor: Fix decoding of empty LZMA2 streams

BugLink: http://bugs.launchpad.net/bugs/780546

commit 646032e3b05b32d3f20cb108a030593d9d792eb5 upstream.

The old code considered valid empty LZMA2 streams to be corrupt.
Note that a typical empty .xz file has no LZMA2 data at all,
and thus most .xz files having no uncompressed data are handled
correctly even without this fix.

Signed-off-by: Lasse Collin <lasse.collin@tukaani.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>