Providing custom start value causes double free

When I provide a custom start value for some of my variables via JuMP, then the solver crashes with segfault and/or double free errors. I can only reproduce for indicator constraints as reported below:

```julia
using Cbc: Cbc
using JuMP: JuMP, @constraint, @objective, @variable

model = JuMP.Model(Cbc.Optimizer)
@variable(model, x[1:2], Bin, start = false)
@variable(model, y[1:2], Bin, start = false)

@constraint(model, x[1] => {y[1] == true})
@constraint(model, x[1] => {y[2] == false})
@constraint(model, x[2] => {y[1] == false})
@constraint(model, x[2] => {y[2] == true})

@objective(model, Max, sum(x))
JuMP.optimize!(model)
```