Because intmap_after_() would simply examine the critbits to walk the
tree, it wouldn't realize that it might be in the completely wrong tree.

In this case:

         Bit 4:
         0   1
        /     \
       /       \
  100000011  100001011

When we ask for intmap_after_(011111111) we would check the critbit, it's
a 1, so we end up on the right leaf instead of the left.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>