Fix Perish Song bug in Doubles

In Doubles, there's the possibility that battle.sides[i].active[j] 
does not exist, thus causing a crash. Add a check for that.
A non-existing Pokémon is not counted for result.
If there's no active Pokémon, there's no field Perish Song 
effect whatsoever.
GP 1/2
Unban Darkrai in STABmons
Merge remote-tracking branch 'upstream/master'