Make sure BattleStream only calls _destroy once

[scheibo]

Fixes Error: Push after end of read stream from #5370 (comment).

this.push(null) triggers this._end() which calls this._destroy() already.

This was fine before (though redundant), but no longer null-ing out this.battle (either by default or through {retainBattle: true}) causes the double _destroy to become a problem because the if (this.battle) this.battle.destroy() check no longer works, and Battle#destroy() is not meant to be called more than once.

[Zarel]

I guess this turned out to be more complicated than you expected?

[scheibo]

I guess this turned out to be more complicated than you expected?

OK, so whats confusing me is this._end also does a this.push(null), which scared me and made me think my reasoning about this.push(null) triggering this._end() was wrong, but I looked it over and that still appears to be the case.

Because there's no way to switch an in progress PR to a draft to indicate I was having second thoughts I closed to try and prevent an accidental merge for code I wasn't sure about.

[Zarel]

Wasn't this line also removed? Am I thinking of a different stream or did this never get merged or what?

[scheibo]

You're thinking of c45ff0e on #5370 which has not been merged into HEAD yet. I just redid those changes on this PR because its not like they were specific to #5370, they just were discovered there, and landing them earlier seems useful given all my PRs involving the harness seem to get bogged down :)