PR libstdc++/83982 fix exception-safety guarantee of std::vector::resize

PR libstdc++/83982 fix exception-safety guarantee of std::vector::resize

Construct new elements before moving existing ones, so that if a default
constructor throws, the existing elements are not left in a moved-from
state.

2018-06-14  Daniel Trebbien <dtrebbien@gmail.com>
	    Jonathan Wakely  <jwakely@redhat.com>

	PR libstdc++/83982
	* include/bits/vector.tcc (vector::_M_default_append(size_type)):
	Default-construct new elements before moving existing ones.
	* testsuite/23_containers/vector/capacity/resize/strong_guarantee.cc:
	New.

Co-Authored-By: Jonathan Wakely <jwakely@redhat.com>
From-SVN: r261585