qcow2: Unaligned zero cluster in handle_alloc()

We should check whether the cluster offset we are about to use is
actually valid; that is, whether it is aligned to cluster boundaries.

Reported-by: R. Nageswara Sastry <nasastry@in.ibm.com>
Buglink: https://bugs.launchpad.net/qemu/+bug/1728643
Buglink: https://bugs.launchpad.net/qemu/+bug/1728657
Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20171110203111.7666-3-mreitz@redhat.com
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
