pimd: Fix core when bouncing interfaces

pimd: Fix core when bouncing interfaces

When you bounce interfaces and the system is under heavy
load there exists situations where zebra may or may not
have told us the new ifindex, but we've requested a
nexthop lookup, which bypasses normal mechanisms for
zebra communication, we may get back a ifindex that
we may have not created a ifp for yet.

Ticket: CM-14052
Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>
Reviewed-by: Don Slice <dslice@cumulusnetworks.com>