qcow2: Prevent numerical overflow

In qcow2_alloc_cluster_offset(), *num is limited to
INT_MAX >> BDRV_SECTOR_BITS by all callers. However, since remaining is
of type uint64_t, we might as well cast *num to that type before
performing the shift.

Cc: qemu-stable@nongnu.org
Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
