PR libstdc++/91910 fix data race in Debug Mode destructors

PR libstdc++/91910 fix data race in Debug Mode destructors

Fix data race when _Safe_iterator_base::_M_detach() runs concurrently with
the _Safe_container_base destructor.

	PR libstdc++/91910
	* src/c++11/debug.cc (_Safe_iterator_base::_M_detach()): Load pointer
	atomically and lock the mutex before accessing the sequence.
	(_Safe_local_iterator_base::_M_detach()): Likewise.
	(_Safe_iterator_base::_M_reset()): Clear _M_sequence atomically.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@276184 138bc75d-0d04-0410-961f-82ee72b054a4