[show] Fail to show interface status after creating portchannel if portchannel members don't speed information.

[chenkelly]

- What I did
The port may not have speed information, we should consider this case when we calculate portchannel speed.
- How I did it
If all portchannel members don't speed information, display N/A

- How to verify it

root@sonic:~# show interfaces status
  Interface    Lanes    Speed    MTU      Alias    Vlan    Oper    Admin            Type    Asym PFC
-----------  -------  -------  -----  ---------  ------  ------  -------  --------------  ----------
  Ethernet0       13      N/A   9100   tenGigE0  routed      up       up  SFP/SFP+/SFP28         N/A
  Ethernet1       14      N/A   9100   tenGigE1  routed      up       up  SFP/SFP+/SFP28         N/A
... 

root@sonic:~# config portchannel add PortChannel1
root@sonic:~# config portchannel member add PortChannel1 Ethernet0
root@sonic:~# config portchannel member add PortChannel1 Ethernet1

- Previous command output (if the output of a command-line utility has changed)

root@sonic:~# show interfaces status
Traceback (most recent call last):
File "/usr/bin/intfutil", line 424, in <module>
main(sys.argv[1:])
File "/usr/bin/intfutil", line 417, in main
interface_stat = IntfStatus(intf_name)
File "/usr/bin/intfutil", line 345, in _init_
self.portchannel_speed_dict = po_speed_dict(self.po_int_dict, self.appl_db)
File "/usr/bin/intfutil", line 231, in po_speed_dict
interface_speed = '{}G'.format(interface_speed[:-3])
TypeError: 'NoneType' object has no attribute '_getitem_'

- New command output (if the output of a command-line utility has changed)

root@sonic:~# show interfaces status
   Interface    Lanes    Speed    MTU      Alias          Vlan    Oper    Admin            Type    Asym PFC
------------  -------  -------  -----  ---------  ------------  ------  -------  --------------  ----------
   Ethernet0       13      N/A   9100   tenGigE0  PortChannel1      up       up  SFP/SFP+/SFP28         N/A
   Ethernet1       14      N/A   9100   tenGigE1  PortChannel1      up       up  SFP/SFP+/SFP28         N/A
......
PortChannel1      N/A      N/A   9100        N/A        routed      up       up             N/A         N/A

Signed-off-by: kelly_chen@edge-core.com

[chenkelly]

retest this please

[chenkelly]

retest this please

[chenkelly]

@lguohan Could you help to review this modification? Please guide us how to add test case. Thanks very much.