pimd: fix crash unconfiguring rp keepalive timer

pimd: fix crash unconfiguring rp keepalive timer

pimd crashs while unconfigure of rp ka timer as we are trying to access
a yand dnode(suppress timer) which does not exist at the moment.

User just configured rp keepalive timer and not suppress timer,
the yang dnode would not be present. Instead of directly accessing
yang_dnode_get_unit16, first check the yang node exist using
the xpath.

Ticket: #3874971

Testing:

Before:
------
tor-11(config)# no ip pim rp keep-alive-timer 3000
vtysh: error reading from pimd: Success (0)Warning: closing connection to pimd because of an I/O error!

Broadcast message from root@tor-11 (somewhere) (Mon Apr 22 17:29:12 2024):

cumulus-core: Running cl-support for core files "pimd.25467.1713806952.core"

After:
-----
tor-11(config)# no ip pim rp keep-alive-timer 3000
tor-11(config)#

Signed-off-by: Donald Sharp <sharpd@nvidia.com>
Signed-off-by: Vijayalaxmi Basavaraj <vbasavaraj@nvidia.com>
(cherry picked from commit 0c4b7c1f1f359c1d7647fea34f62378af8b640a5)