Fix ignored output from user function in left side of assignment (bug #59704). * libinterp/parse-tree/pt-assign.cc (tree_multi_assignment::evaluate_n): Set lvalue list of tree_evaluator only to process the right side of the assignment. Reset it to null after that. * test/bug-59704/bug59704_2.m, test/bug-59704/bug-59704.tst, test/bug-59704/module.mk: Add new test case.

%!test
%! o = bug59704_1 ();
%! o.test ();
%! assert (o.p, [])

%!test
%! o = bug59704_2 ();
%! [~, o(1)] = size (rand (2, 5));
%! assert (o(1), 5)