Fix child-address counting.

Fix child-address counting.

When a new address was created by a routing step it was possible for the parent address in the tree to
be marked as having zero children, despite the new child having a pointer to the parent.  When the child
was then delivered, the count on the parent could go negative or, if other children had been added which
correctly incremented the count, arrive at zero while some children were outstanding.  Fix this to
maintin the invariant.  While there, make the counter unsigned.

(cherry picked from commit 82f90600647a5322e9e7b58fc127eb8be839165c)
Signed-off-by: Phil Pennock <pdp@exim.org>