Corrected item bonus FixedCastRate (#3808)

* Fixes #3804.
* Corrected FixedCastRate attempting to save into VariableCastRate bonus array.
Thanks to @laziem!

src/map/pc.cpp

@@ -3713,7 +3713,7 @@ void pc_bonus2(struct map_session_data *sd,int type,int type2,int val)
 			break;
 		}
 
-		pc_bonus_itembonus(sd->skillcastrate, type2, -val); // Send inversed value here
+		pc_bonus_itembonus(sd->skillfixcastrate, type2, -val); // Send inversed value here
 		break;
 #else
 	case SP_SKILL_FIXEDCAST: // bonus2 bSkillFixedCast,sk,t;