Fix to #790: Ping errors solved.

[TreadTheDawnGames]

Fixed the invalid index exception described in #790.

Pings will now snap to nearest valid item (i.e. asteroid if in space.) Added a check to ensure ping location is valid. Only draw pings if its location is logically visible. (If in space you can see the pings on the planet, if on the planet you cannot see pings in orbit.)