Do not crash on bogus ipmitool output

We have a TrueNAS HA system that IT is using and the passive controller is experiencing hardware related issues. In the webUI, a traceback is being generated.

**Standby Controller - Unable to run alert source 'Sensors' Traceback (most recent call last): File "/usr/local/lib/python3.6/site-packages/middlewared/plugins/alert.py", line 469, in __run_source alerts = (await alert_source.check()) or [] File "/usr/local/lib/python3.6/site-packages/middlewared/alert/base.py", line 100, in check return await self.middleware.run_in_thread(self.check_sync) File "/usr/local/lib/python3.6/site-packages/middlewared/main.py", line 1006, in run_in_thread return await self.loop.run_in_executor(executor, functools.partial(method, *args, **kwargs)) File "/usr/local/lib/python3.6/concurrent/futures/thread.py", line 56, in run result = self.fn(*self.args, *self.kwargs) File "/usr/local/lib/middlewared_truenas/alert/source/sensors.py", line 98, in check_sync for sensor in sensor_list(): File "/usr/local/lib/middlewared_truenas/alert/source/sensors.py", line 48, in sensor_list if fields[index] is None: IndexError: list index out of range*