Topology:
A single router is acting as the First Hop Router (FHR), Last Hop Router (LHR), and RP.

RC and Issue:
When an upstream S,G is in join state, it sends a register message to the RP.
If the RP has the receiver, it sends a register stop message and switches to the shortest path.
When the register stop message is processed, it removes pimreg, moves to prune,
and starts the reg stop timer.

When the reg stop timer expires, PIM changes S,G state to Join Pending and sends out a NULL
register message to RP. RP receives it and fails to send Reg stop because SPT is not set at that point.

The problem is when the register stop timer pops and state is in Join Pending.
According to https://www.rfc-editor.org/rfc/rfc4601#section-4.4.1,
we need to put back the pimreg reg tunnel into the S,G mroute.
This causes data to be sent to the control plane and subsequently interrupts the line rate.

Fix:
If the router is FHR and RP to the group,
ignore SPT status and send out a register stop message back to the DR (in this context, the same router).

Ticket: #3506780

Signed-off-by: Donald Sharp <sharpd@nvidia.com>
Signed-off-by: Rajesh Varatharaj <rvaratharaj@nvidia.com>
(cherry picked from commit 8280257)