libstdc++: Add assertions for preconditions in sampling distributions [PR 82584]

libstdc++: Add assertions for preconditions in sampling distributions [PR 82584]

These three distributions all require 0 < S where S is the sum of the
weights. When the sum is zero there's an undefined FP division by zero.
Add assertions to help users diagnose the problem.

libstdc++-v3/ChangeLog:

	PR libstdc++/82584
	* include/bits/random.tcc
	(discrete_distribution::param_type::_M_initialize)
	(piecewise_constant_distribution::param_type::_M_initialize)
	(piecewise_linear_distribution::param_type::_M_initialize):
	Add assertions for positive sums..
	* testsuite/26_numerics/random/pr60037-neg.cc: Adjust dg-error
	line.