bridge: avoid OOPS if root port not found

Bridge can crash while trying to send topology change packet.
This happens if root port can't be found. This was reported by user
but currently unable to reproduce it easily. The STP conditions that cause
this are not known yet, but the problem doesn't have to be fatal.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>