isisd: ignore unknown interfaces when adjusting IS-IS mtu

isisd: ignore unknown interfaces when adjusting IS-IS mtu

For example during startup of isisd, the MTU of interfaces is not
known, since this information will only be available once the
interfaces have been learned from zebra.

It makes no sense to include the MTU 0 that is stored for interfaces
in this state in the consideration whether a new lsp-mtu for an
area is valid, so skip interfaces which are in this state.

Signed-off-by: Christian Franke <nobody@nowhere.ws>