Don't lose trade gain when ordering a trader.

Introduced in r25216 / rP25216 due to overeagerly stopping trade when leaving the trading-state.
Noticed in: https://wildfiregames.com/forum/topic/41264-alpha-25-pre-releaserelease-candidate-build-testing/page/9/?tab=comments#comment-437888.

Differential revision: https://code.wildfiregames.com/D4197
Comment by: @Stan