Fixes material market prices going negative & runtime when buying materials (#79732)

## About The Pull Request
- Fixes #79114
- Fixes #79245

The hope is that it should fix these 2 issues because looking at the
math is seems solid. This PR ensures 2 things
- If the current market price of a material is 0 then neither the market
quantity nor its price is changed because the material is worthless at
that point so adding more is just meaningless
- Corrects the formulae to calculate the new price so we get the right
rate of change

For more details on how this bug occurs refer to
https://github.com/tgstation/tgstation/issues/79114#issuecomment-1790757814

Mentioning @Iajret in the changelog cause credits(no pun intended) goes
to them for discovering said bug

This also corrects a runtime that went unresolved in
- #79307

I could not get my code suggestion there in time so i'm just carrying it
over here

## Changelog
SyncIt21, Iajret
🆑
fix: selling large amount of mats in cargo should not give you infinite
credits
fix: runtime when adjusting material market after buying 
/🆑

code/modules/cargo/exports/materials.dm

@@ -134,13 +134,24 @@
 /datum/export/material/market/sell_object(obj/sold_item, datum/export_report/report, dry_run, apply_elastic)
 	. = ..()
 	var/amount = get_amount(sold_item)
-	var/price = get_cost(sold_item)
 	if(!amount)
 		return
+
+	//This formula should impact lower quantity materials greater, and higher quantity materials less. Still, it's  a bit rough. Tweaking may be needed.
 	if(!dry_run)
+		//this material is worthless. no point adding more to the stock
+		var/market_price = SSstock_market.materials_prices[material_id]
+		if(!market_price)
+			return
+
+		//decrease the market price
+		market_price -= round((market_price) * (amount / (amount + SSstock_market.materials_quantity[material_id])))
+		if(market_price < 0)
+			market_price = 0
+		SSstock_market.materials_prices[material_id] = market_price
+
+		//increase the stock
 		SSstock_market.materials_quantity[material_id] += amount
-		SSstock_market.materials_prices[material_id] -= round((price) * (amount / (amount + SSstock_market.materials_quantity[material_id])))
-		//This formula should impact lower quantity materials greater, and higher quantity materials less. Still, it's  a bit rough. Tweaking may be needed.
 
 
 // Stock blocks are a special type of export that can be used to sell a quantity of materials at a specific price on the market.

code/modules/cargo/packs/_packs.dm

@@ -124,14 +124,28 @@
 	src.contains = contains
 
 /datum/supply_pack/custom/minerals/fill(obj/structure/closet/crate/C)
-	. = ..()
-	//Remove our material sheets from SSstock_market's materials_quantity equal to the quantity within the crate.
 	for(var/obj/item/stack/sheet/possible_stack as anything in contains)
-		if(!ispath(possible_stack, /obj/item/stack/sheet))
+		var/material_type = initial(possible_stack.material_type)
+		//in case we ordered more than what's in the market at the time due to market fluctuations
+		//we find the min of what was ordered & what's actually available in the market at this point of time
+		var/market_quantity = SSstock_market.materials_quantity[material_type]
+		var/available_quantity = min(contains[possible_stack], market_quantity)
+		if(!available_quantity)
 			continue
-		if(!possible_stack.material_type)
-			continue
-		if(!SSstock_market.materials_quantity[possible_stack.material_type])
-			continue
-		SSstock_market.materials_quantity[possible_stack.material_type] -= contains[possible_stack]
-		SSstock_market.materials_prices[possible_stack.material_type] += round((SSstock_market.materials_prices[possible_stack.material_type]) * (contains[possible_stack] / (SSstock_market.materials_quantity[possible_stack.material_type] - contains[possible_stack])))
+
+		//spawn the ordered stack inside the crate
+		var/sheets_to_spawn = available_quantity
+		while(sheets_to_spawn)
+			var/spawn_quantity = min(sheets_to_spawn, MAX_STACK_SIZE)
+			var/obj/item/stack/sheet/ordered_stack = new possible_stack(C, spawn_quantity)
+			if(admin_spawned)
+				ordered_stack.flags_1 |= ADMIN_SPAWNED_1
+			sheets_to_spawn -= spawn_quantity
+
+		//Prices go up as material quantity becomes scarce
+		var/fraction = available_quantity
+		if(market_quantity != available_quantity) //to avoid division by zero error
+			fraction /= (market_quantity - available_quantity)
+		SSstock_market.materials_prices[material_type] += round(SSstock_market.materials_prices[material_type] * fraction)
+		//We decrease the quantity only after adjusting our prices for accurate values
+		SSstock_market.materials_quantity[material_type] -= available_quantity