pimd: Modify the Prune Pending Timer Pop to not assert

pimd: Modify the Prune Pending Timer Pop to not assert

So there exist conditions where we can start the Prune
Pending Timer and receive other packets that cause
us to not stop the pending timer.  This was
due to a missread of the state machine.

Additionally when the prune pending timer pops and
we are not in prune pending state, note the fact
and move on with our life instead of crashing and burning

Ticket: CM-13851
Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>
Reviewed-by: