libstdc++: Fix unsafe comma operators in <random> [PR122062]

libstdc++: Fix unsafe comma operators in <random> [PR122062]

This fixes a 'for' loop in std::piecewise_linear_distribution that
increments two iterators with a comma operator between them, making it
vulnerable to evil overloads of the comma operator.

It also changes a 'for' loop used by some other distributions, even
though those are only used with std::vector<double>::iterator and so
won't find any overloaded commas.

libstdc++-v3/ChangeLog:

	PR libstdc++/122062
	* include/bits/random.tcc (__detail::__normalize): Use void cast
	for operands of comma operator.
	(piecewise_linear_distribution): Likewise.
	* testsuite/26_numerics/random/piecewise_linear_distribution/cons/122062.cc:
	New test.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>
Reviewed-by: Hewill Kang <hewillk@gmail.com>