Error in Modules/mavproxy_arm.py handling ICE_START_CHAN=0 on arm and disarm

The documentation within AP_ICEngine.cpp states that a value of 0 corresponds to "None" for the ICE_START_CHAN, and thus should be valid but effectively treat the "start" channel as always being the middle value.
mavproxy_arm.py throws the following error when you arm or disarm with ICE_START_CHAN set to 0:
`Got MAVLink msg: COMMAND_ACK {command : 400, result : 0}
DISARMED
Traceback (most recent call last):
File "MAVProxy\modules\mavproxy_link.py", line 625, in master_callback
File "MAVProxy\modules\mavproxy_arm.py", line 168, in mavlink_packet
AttributeError: 'MAVLink_rc_channels_message' object has no attribute 'chan0_raw'`

The code in questions is from line 208 onwards:
```
if ice_enable == 1:
rc = self.master.messages["RC_CHANNELS"]
v = self.mav_param.get('ICE_START_CHAN', None)
if v is None:
return
v = getattr(rc, 'chan%u_raw' % v)
if v <= 1300:
self.say("ICE Disabled")
```

I think in addition to the check for "None", it should also check for v=0 and return, rather than trying to read "chan0u_raw".

Start in Modules/mavproxy_arm.py around line 208, where mavlink_packet reads ICE_START_CHAN and accesses the RC_CHANNELS message. Reproduce arming or disarming with ICE_START_CHAN set to 0, then verify the handling no longer attempts to access chan0_raw and completes without the reported AttributeError while retaining the existing None behavior.

Written by the indexing model from the issue text.