[1841] allows EMR when corp can afford a discarded train

[philcampeau]

https://18xx.games/game/167460?action=445

How to Reproduce the Bug?

At this step, the corporation was trainless with L.616 in its treasury. No shares to issue. There was a L.350 4-train discarded in the bank pool, and the next available train was an L.800 6-train.

The player was able to click Buy on the 6-train and contribute the remaining balance. This seems to contradict the EMR rules from the rulebook:

[philcampeau]

I noticed that in the log, it mentioned [19:59] Sweeping L.184 from Eonthar to SFL (EMR). Looking at the code, it looks like the emergency_assist.rb code is being triggered for EMR, but I suspect that wasn't intended.