Don't crash when spell school level is higher than 3 (which is another bug)

lib/mapObjects/CGHeroInstance.cpp

@@ -865,7 +865,7 @@ ui8 CGHeroInstance::getSpellSchoolLevel(const CSpell * spell, int *outSelectedSc
 	
 	spell->forEachSchool([&, this](const SpellSchoolInfo & cnf, bool & stop)
 	{
-		int thisSchool = std::max<int>(getSecSkillLevel(cnf.skill),	valOfBonuses(Bonus::MAGIC_SCHOOL_SKILL, 1 << ((ui8)cnf.id))); 
+		int thisSchool = std::max<int>(getSecSkillLevel(cnf.skill),	valOfBonuses(Bonus::MAGIC_SCHOOL_SKILL, 1 << ((ui8)cnf.id))); //FIXME: Bonus shouldn't be additive (Witchking Artifacts : Crown of Skies)
 		if(thisSchool > skill)									
 		{														
 			skill = thisSchool;									
@@ -877,7 +877,8 @@ ui8 CGHeroInstance::getSpellSchoolLevel(const CSpell * spell, int *outSelectedSc
 	vstd::amax(skill, valOfBonuses(Bonus::MAGIC_SCHOOL_SKILL, 0)); //any school bonus
 	vstd::amax(skill, valOfBonuses(Bonus::SPELL, spell->id.toEnum())); //given by artifact or other effect
 
-	assert(skill >= 0 && skill <= 3);
+	vstd::amax(skill, 0); //in case we don't know any school
+	vstd::amin(skill, 3);
 	return skill;
 }