isisd: remove assert from the TI-LFA repair list computation algorithm

isisd: remove assert from the TI-LFA repair list computation algorithm

In some cases it's possible that the TI-LFA algorithms will try to
compute a SID repair list more than once for the same backup nexthop
[1]. This of course shouldn't be allowed, as a backup nexthop can't
have multiple label stacks. When that happens, we should just ignore
the new repair list if one is already applied, instead of asserting
and crashing the daemon.

[1] One scenario this can happen is when there's ECMP involving
different P-nodes in the PQ-space intersection.

Reported-by: Fredi Raspall <fredi@voltanet.io>
Signed-off-by: Renato Westphal <renato@opensourcerouting.org>