ripd: unlink if-rmap container from global list before removing it

ripd: unlink if-rmap container from global list before removing it

This solves a crash that happens if the "route-map" command is used
after "router rip" + "no router rip" + "router rip".

Once interface route-maps are converted to the new northbound model,
we'll be able to remove the if_rmap_ctx_list global list (which is
an ugly hack to make things work right now).

Bug found by the CLI fuzzer.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>