Fix ret.prune not initialized.

To leverage dual pruning, the current implementation
uses a struct called relax_info as a return for l1 algorithms functions.
This relax_info contains, among other things, whether the node must be pruned
or not.
This structure was created simply with a:
```
relax_info ret
```
And then, if we are in the correct case, ret.prune is set to true.
This assumes the above declaration initialize ret.prune to false.
This is not the case for every compiler-os-platform combination.
This commit fixes this by forcing the initialization to false.