libstdc++: Fix bogus -Wstringop-overflow in std::vector::insert [PR117983]

libstdc++: Fix bogus -Wstringop-overflow in std::vector::insert [PR117983]

This was fixed on trunk by r15-4473-g3abe751ea86e34, but that isn't
suitable for backporting. Instead, just add another unreachable
condition in std::vector::_M_range_insert so the compiler knows this
memcpy doesn't use a length originating from a negative ptrdiff_t
converted to a very positive size_t.

libstdc++-v3/ChangeLog:

	PR libstdc++/117983
	* include/bits/vector.tcc (vector::_M_range_insert): Add
	unreachable condition to tell the compiler begin() <= end().
	* testsuite/23_containers/vector/modifiers/insert/117983.cc: New
	test.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>
(cherry picked from commit 878812b6f6905774ab37cb78903e3e11bf1c508c)