Fix various code issues found by cppcheck (#7741)

Mapgen Singlenode: Remove 'flags' that duplicates a 'class Mapgen' member.
Dungeongen: Initialise 'MMVManip *vm' to 'nullptr'.
Inventorymanager: Remove assignment error 'found = '.

src/inventorymanager.cpp

@@ -846,7 +846,7 @@ void ICraftAction::apply(InventoryManager *mgr,
			count_remaining--;

		// Get next crafting result
		found = getCraftingResult(inv_craft, crafted, temp, false, gamedef);
		getCraftingResult(inv_craft, crafted, temp, false, gamedef);
		PLAYER_TO_SA(player)->item_CraftPredict(crafted, player, list_craft, craft_inv);
		found = !crafted.empty();
	}

src/mapgen/dungeongen.h

@@ -64,7 +64,7 @@ struct DungeonParams {

class DungeonGen {
public:
	MMVManip *vm;
	MMVManip *vm = nullptr;
	const NodeDefManager *ndef;
	GenerateNotifier *gennotify;

src/mapgen/mapgen_singlenode.cpp

@@ -33,8 +33,6 @@ MapgenSinglenode::MapgenSinglenode(int mapgenid,
	MapgenParams *params, EmergeManager *emerge)
	: Mapgen(mapgenid, params, emerge)
{
	flags = params->flags;

	const NodeDefManager *ndef = emerge->ndef;

	c_node = ndef->getId("mapgen_singlenode");

src/mapgen/mapgen_singlenode.h

@@ -35,7 +35,6 @@ struct MapgenSinglenodeParams : public MapgenParams
class MapgenSinglenode : public Mapgen
{
public:
	u32 flags;
	content_t c_node;
	u8 set_light;