Prevent nonsense accuracies from being shown on the navigation page

[marikavertzonis]

The accuracy shown in this screenshot is not useful. It might supposed to be indicating a poor or false accuracy, but it will be better to show no accuracy than this number. This was found on build 3.0.17

[dominikargast]

@marikavertzonis test with 3.0.18

[marikavertzonis]

tested 3.0.18 with same receiver (emtac bt gps) and the trimble tdc and i see no more extra large numbers.