ofproto: Check ofproto_port_query_by_name() return value when adding port.

ofproto: Check ofproto_port_query_by_name() return value when adding port.

Otherwise, if the port add succeeds but the query that looks up the port
number fails, then ofproto_port_add() would return zero as the OpenFlow
port number and ignore the error.

Reported-by: Guolin Yang <gyang@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>