vswitchd: Avoid segfault when local port is required but missing.

vswitchd: Avoid segfault when local port is required but missing.

Connecting to a controller requires the vswitch to have a local port
(typically named the same as the bridge itself).  Before the introduction
of ovsdb ovs-vswitchd simply added the local port itself if it was missing.
This was not properly implemented with the ovsdb transition, and a segfault
resulted.  This commit avoids the segfault, although it should possibly
be improved to also add the local port, as before.

CC: Jeremy Stribling <strib@nicira.com>