(svn r15541) -Revert (r15399): 'v->cargo_type' is also used in other places, which cannot accept CT_INVALID.

-Codechange: Add Engine::GetDefaultCargoType() and Engine::CanCarryCargo() and use them.
-Fix [FS#2617]: When articulated parts have no available default cargo, use the cargo type of the first part for livery selection.
-Change: To decide whether a vehicle is refittable do not test its current capacity for being zero, but always use the 'capacity property'.
  Note: The property is used unmodifed without calling CB 15/36. By setting it to a non-zero value and returning zero in the callback vehicles can be refitted to/from zero capacity for e.g. livery effects.
  Note: It is intentional that you cannot control refittability by CB 36.

src/ship_cmd.cpp

@@ -737,6 +737,9 @@ CommandCost CmdBuildShip(TileIndex tile, DoCommandFlag flags, uint32 p1, uint32
 	const Engine *e = GetEngine(p1);
 	CommandCost value(EXPENSES_NEW_VEHICLES, e->GetCost());
 
+	/* Engines without valid cargo should not be available */
+	if (e->GetDefaultCargoType() == CT_INVALID) return CMD_ERROR;
+
 	if (flags & DC_QUERY_COST) return value;
 
 	/* The ai_new queries the vehicle cost before building the route,
@@ -772,7 +775,7 @@ CommandCost CmdBuildShip(TileIndex tile, DoCommandFlag flags, uint32 p1, uint32
 		v->vehstatus = VS_HIDDEN | VS_STOPPED | VS_DEFPAL;
 
 		v->spritenum = svi->image_index;
-		v->cargo_type = svi->cargo_type;
+		v->cargo_type = e->GetDefaultCargoType();
 		v->cargo_subtype = 0;
 		v->cargo_cap = svi->capacity;
 		v->value = value.GetCost();