PR tree-optimization/83239 - False positive from -Wstringop-overflow
on simple std::vector code

libstdc++/CHangeLog:
	* include/bits/vector.tcc (vector::_M_default_append): Assert
        invariant to generate better code.

gcc/testsuite/ChangeLog:
	* g++.dg/pr83239.C: New test case.

From-SVN: r255753