Make getStackMax return the correct maximal stack size

src/inventory.h

@@ -80,15 +80,14 @@ struct ItemStack
 	// Maximum size of a stack
 	u16 getStackMax(IItemDefManager *itemdef) const
 	{
-		s16 max = itemdef->get(name).stack_max;
-		return (max >= 0) ? max : 0;
+		return itemdef->get(name).stack_max;
 	}
 
 	// Number of items that can be added to this stack
 	u16 freeSpace(IItemDefManager *itemdef) const
 	{
 		u16 max = getStackMax(itemdef);
-		if(count > max)
+		if (count >= max)
 			return 0;
 		return max - count;
 	}

src/itemdef.h

@@ -61,7 +61,7 @@ struct ItemDefinition
 	/*
 		Item stack and interaction properties
 	*/
-	s16 stack_max;
+	u16 stack_max;
 	bool usable;
 	bool liquids_pointable;
 	// May be NULL. If non-NULL, deleted by destructor

src/script/common/c_content.cpp

@@ -65,9 +65,8 @@ ItemDefinition read_item_definition(lua_State* L,int index,
 	}
 	lua_pop(L, 1);
 
-	def.stack_max = getintfield_default(L, index, "stack_max", def.stack_max);
-	if(def.stack_max == 0)
-		def.stack_max = 1;
+	int stack_max = getintfield_default(L, index, "stack_max", def.stack_max);
+	def.stack_max = rangelim(stack_max, 1, U16_MAX);
 
 	lua_getfield(L, index, "on_use");
 	def.usable = lua_isfunction(L, -1);