timer: fix two corruption bugs.

We fill the upper buckets after we've moved on from the first bucket
in this layer.  This means two things:
1) If we have to look at an upper layer, we need to look at the next bucket,
   unless offset is 0.
2) We need to keep looking up layers in the corner case, too.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>