Fix: Add nullcheck for linkedGoods (ShadowTheAge#193)

While using YaFC-CE today i ran into an exception because `linkedGoods`
in `summer.TryGetValue(linkedGoods, ...)` was null. This fixed that.

It's kinda a blind fix since i have no steps to reproduce, YaFC was
hanging right after the exception message (i made another issue for
that). But judging the code this should work.

Yafc.Model/Model/ProductionTable.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using System.Runtime.CompilerServices;
@@ -183,9 +184,14 @@ private static void AddFlow(RecipeRow recipe, Dictionary<Goods, (double prod, do
             for (int i = 0; i < recipe.recipe.ingredients.Length; i++) {
                 var ingredient = recipe.recipe.ingredients[i];
                 var linkedGoods = recipe.links.ingredientGoods[i];
-                _ = summer.TryGetValue(linkedGoods, out var prev);
-                prev.cons += recipe.recipesPerSecond * ingredient.amount;
-                summer[linkedGoods] = prev;
+                if (linkedGoods is not null) {
+                    _ = summer.TryGetValue(linkedGoods, out var prev);
+                    prev.cons += recipe.recipesPerSecond * ingredient.amount;
+                    summer[linkedGoods] = prev;
+                }
+                else {
+                    Debug.WriteLine("linkedGoods should not have been null here.");
+                }
             }
 
             if (recipe.fuel != null && !float.IsNaN(recipe.parameters.fuelUsagePerSecondPerBuilding)) {