Unfortunately it seems that I didn't properly test the case of
an expired external querier in the recent multicast bridge series.

The setup of the timer in that case is completely broken and leads
to a NULL-pointer dereference.  This patch fixes it.
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>
Acked-by: NStephen Hemminger <shemminger@vyatta.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>