PR libstdc++/81891 fix double-free in hashtable constructor

PR libstdc++/81891 fix double-free in hashtable constructor

Backport from mainline
2017-08-18  Jonathan Wakely  <jwakely@redhat.com>

	PR libstdc++/81891
	* include/bits/hashtable.h (_Hashtable(_InputIterator, _InputIterator,
	size_type, const _H1&, const _H2&, const _Hash&, const _Equal&,
	const _ExtractKey&, const allocator_type&)): Let destructor do clean
	up if an exception is thrown.
	* testsuite/23_containers/unordered_map/cons/81891.cc: New.

From-SVN: r251671