Add error checking around ping squawk value to avoid lockup

We have a report that MP locksup if and ADSB ping's "squawk" value is set to be out of range (e.g. 1200 and 7777). We should add better error checking around what the user sets this to to avoid the lockup.

Start by locating the Mission Planner handling for ADS-B ping squawk values and reproduce the reported out-of-range inputs, such as 1200 and 7777. Confirm the current behavior and identify the relevant validation path. Done means invalid squawk values are handled without locking up Mission Planner.

Written by the indexing model from the issue text.