pull-hook: no-op on double add
If we receive an %add poke for a resource we are already pulling, no-op
instead of crashing. This should prevent crashes upon repulling a
resource after a breach of the host.
++ add
|= [=ship =resource]
~| resource
- ?< (~(has by tracking) resource)
+ ?: (~(has by tracking) resource)
+ [~ state]
=. tracking
(~(put by tracking) resource ship)
:_ state
