block/qcow2-threads: fix qcow2_decompress

block/qcow2-threads: fix qcow2_decompress

On success path we return what inflate() returns instead of 0. And it
most probably works for Z_STREAM_END as it is positive, but is
definitely broken for Z_BUF_ERROR.

While being here, switch to errno return code, to be closer to
qcow2_compress API (and usual expectations).

Revert condition in if to be more positive. Drop dead initialization of
ret.

Cc: qemu-stable@nongnu.org # v4.0
Fixes: 341926ab83e2b
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20200302150930.16218-1-vsementsov@virtuozzo.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>