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: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>