boss: make extra spell trigger condition more robust

Fixes #377

src/boss.c

@@ -169,13 +169,10 @@ static StageProgress *get_spellstage_progress(Attack *a, StageInfo **out_stginfo
 }
 
 static bool boss_should_skip_attack(Boss *boss, Attack *a) {
- if(a->type == AT_ExtraSpell || (a->info != NULL && a->info->type == AT_ExtraSpell)) {
- if(global.plr.voltage < global.voltage_threshold) {
- return true;
- }
- }
-
  // Skip zero-length spells. Zero-length AT_Move and AT_Normal attacks are ok.
+ // FIXME: I'm really not sure what was the purpose of this, but for now I'm abusing this to
+ // conditionally flag the extra spell as skipped. Investigate whether we can remove simplify
+ // things a bit and remove this function.
  if(ATTACK_IS_SPELL(a->type) && a->timeout <= 0) {
  return true;
  }
@@ -1068,6 +1065,17 @@ void boss_finish_current_attack(Boss *boss) {
  );
  }
 
+ if(boss->current < boss->attacks + boss->acount - 1) {
+ // If the next attack is an extra spell, determine whether we have enough voltage now, and
+ // if not, skip it. This can't be done any later, because we have to know whether to start
+ // the death sequence this frame (since the extra spell is usually the final one).
+ Attack *next = boss->current + 1;
+ if(next->type == AT_ExtraSpell && global.plr.voltage < global.voltage_threshold) {
+ // see boss_should_skip_attack()
+ next->timeout = 0;
+ }
+ }
+
  boss->current->endtime = global.frames + attack_end_delay(boss);
  boss->current->endtime_undelayed = global.frames;
 
@@ -1267,7 +1275,6 @@ void process_boss(Boss **pboss) {
  }
 
  boss->current++;
- assert(boss->current != NULL);
 
  if(boss_should_skip_attack(boss, boss->current)) {
  COEVENT_CANCEL_ARRAY(boss->current->events);