ofproto-dpif: Fix segfault removing port when STP is enabled.

Reported-by: Sridhar Samudrala <samudrala.sridhar@gmail.com>
Tested-by: Sridhar Samudrala <samudrala.sridhar@gmail.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>