Match the squad check inside .CenterPosition

[PunkPun]

Exception of type `System.InvalidOperationException`: Attempting to query the position of an invalid Target
   at OpenRA.Traits.Target.get_CenterPosition() in /OpenRA/OpenRA.Game/Traits/Target.cs:line 169
   at OpenRA.Mods.Common.Traits.BotModules.Squads.Squad.Serialize() in /OpenRA/OpenRA.Mods.Common/Traits/BotModules/Squads/Squad.cs:line 148
   at OpenRA.Mods.Common.Traits.SquadManagerBotModule.<>c.<OpenRA.Traits.IGameSaveTraitData.IssueTraitData>b__41_0(Squad s) in /OpenRA/OpenRA.Mods.Common/Traits/BotModules/SquadManagerBotModule.cs:line 525
   at System.Collections.Generic.List`1.ConvertAll[TOutput](Converter`2 converter)
   at OpenRA.Mods.Common.Traits.SquadManagerBotModule.OpenRA.Traits.IGameSaveTraitData.IssueTraitData(Actor self) in /OpenRA/OpenRA.Mods.Common/Traits/BotModules/SquadManagerBotModule.cs:line 523

[pchote]

Not tested, but fix is clearly correct (Target.Invalid is specifically set up to not equal itself).

[pchote]

https://github.com/OpenRA/OpenRA/wiki/Changelog/_compare/7691ce7f17f6259dfa24dfd1a798dfa6d7dd8045

9a78f22