pychoco crashes when solving an optimisation problem multiple times

Hi,

I'm trying to solve an optimisation problem multiple times, but I'm running into issues.
As an example see the following code:

```
from pychoco.model import Model

m = Model()
x = m.intvar(1, 10, "x")

sol1 = m.get_solver().find_optimal_solution(objective=x, maximize=False)
print(sol1 is not None, sol1.get_int_val(x)) # True, 1

sol2 = m.get_solver().find_optimal_solution(objective=x, maximize=False)
print(sol2 is not None) # True
print(sol2.get_int_val(x)) # FATAL: empty Solution, crashes
```

As can be seen, after the second call there does seem to be a solution although it is not actually accessible. It might actually be the same issue as #32.