Fix slither-read-storage crash when a structure has only other structs as fields

[smonicas]

Fix a crash when a struct has only other structs. The size variable would not be initialized.

contract T {
  struct A {
   uint256 value;
  }

  struct T {
   A _inner;
  }

  T _t;
}

  File ".../slither/tools/read_storage/read_storage.py", line 597, in _find_struct_var_slot
    return info, type_to, slot_as_bytes, size, offset
                                         ^^^^
UnboundLocalError: cannot access local variable 'size' where it is not associated with a value