Rope iterators: don't retain pointers when copied

Rope iterators: don't retain pointers when copied

Rope iterators sometimes contain pointers to an internal buffer
inside the iterator itself.  When such an iterator is copied, the
copy incorrectly retains pointers to the original.

This patch takes the simple approach of not copying the cached
information when the internal buffer is being used, instead
requiring it to be recomputed when the copied iterator is
dereferenced.  An alternative would be to adjust the pointers so
they refer to the buffer in the copy.

2018-08-14  Jeremy Sawicki  <jeremy-gcc@sawicki.us>

	* include/ext/rope (_Rope_iterator_base(const _Rope_iterator_base&))
	(_Rope_const_iterator::operator=(const _Rope_const_iterator&))
	(_Rope_iterator::operator=(const _Rope_iterator&)): Ensure
	copied/assigned rope iterators don't retain pointers to the iterator
	they were copied/assigned from.
	* testsuite/ext/rope/7.cc: New.

From-SVN: r263534