Gracefully handle resending a node to prune_tree()

Gracefully handle resending a node to prune_tree()

Commit 801e888d03e0ae34c5ecf00385defa77844f4023 made the prune_tree()
function use send_to_prune_tree() for triggering pruning of deleted leaf
nodes' parents.  This enabled the following sequence of events to
happen:

 1. Node A, which is a leaf node, is passed to send_to_prune_tree() and
    its pruning is queued.

 2. Node B is added to the RBTDB as a child of node A before the latter
    gets pruned.

 3. Node B, which is now a leaf node itself (and is likely to belong to
    a different node bucket than node A), is passed to
    send_to_prune_tree() and its pruning gets queued.

 4. Node B gets pruned.  Its parent, node A, now becomes a leaf again
    and therefore the prune_tree() call that handled node B calls
    send_to_prune_tree() for node A.

 5. Since node A was already queued for pruning in step 1 (but not yet
    pruned), the INSIST(!ISC_LINK_LINKED(node, prunelink)); assertion
    fails for node A in send_to_prune_tree().

The above sequence of events is not a sign of pathological behavior.
Replace the assertion check with a conditional early return from
send_to_prune_tree().

(cherry picked from commit f6289ad93141a29443d1e8e9874e36d44f16e686)