[intfutil] set speed to 0 when interface speed is not available (#839)

This is not an issue with normal and correct configuration. The
issue was exposed when there is an incorrect configuration, e.g.
contain wrong port names. These wrong port names will still get
populated to the app_db but will not have speed associated.

Lack of speed entry will cause "show interface status" to throw
exception.

Signed-off-by: Ying Xie <ying.xie@microsoft.com>

scripts/intfutil

@@ -276,7 +276,7 @@ def po_speed_dict(po_int_dict, appl_db):
             elif len(value) > 1:
                 for intf in value:
                     temp_speed = appl_db.get(appl_db.APPL_DB, "PORT_TABLE:" + intf, "speed")
-                    temp_speed = int(temp_speed)
+                    temp_speed = int(temp_speed) if temp_speed else 0
                     agg_speed_list.append(temp_speed)
                     interface_speed = sum(agg_speed_list)
                     interface_speed = str(interface_speed)