Fixing variables on quadratic expression causes memory errors.

[arcondello]

Description
#1319 introduced a bug in `dimod::ConstrainedQuadraticModel::fix_variables()``.

Steps To Reproduce

#include "dimod/constrained_quadratic_model.h"

auto cqm = dimod::ConstrainedQuadraticModel<double>();

cqm.add_variables(dimod::Vartype::BINARY, 5);

auto c = cqm.add_linear_constraint({3, 1, 4, 0}, {1, 2, 3, 4}, dimod::Sense::LE, 5);
cqm.constraint_ref(c).add_quadratic(3, 1, -15);
cqm.constraint_ref(c).add_quadratic(4, 0, 14);
cqm.constraint_ref(c).add_quadratic(4, 3, 17);

std::vector<int> variables{1, 2, 3};
std::vector<double> values{1, 1, 1};

cqm.fix_variables(variables.begin(), variables.end(), values.begin());

will cause memory corruption.