Should fix #254.

It looks like either at some point `info.player` might have
been synonymous for `info.player_index`, or this card could
have never worked OK. `info.player` is `null`, which is being
implicitly promoted to `0`, giving the mana bonus always to
player 0. This explains the misbehavior described by SkippyQ.
I actually checked that this change fixes the problem.