gdbsupport: allow passing nullptr to checked_static_cast

Both static_cast and dynamic_cast handle nullptr (they return nullptr),
so I think checked_static_cast should too.  This will allow doing a null
check after a checked_static_cast:

  cooked_index_vector *table
    = (gdb::checked_static_cast<cooked_index_vector *>
       (per_bfd->index_table.get ()));
  if (table != nullptr)
    return;

Change-Id: If5c3134e63696f8e417c87b5f3901240c9f7ea97