GDScript: Properly validate return type

[vnen]

When the type cannot be validated at compile time, the runtime must do a check to ensure type safety is kept, as the code might be assuming the return type is correct in another place, leading to crashes if the contract is broken.

[qarmin]

When running https://github.com/qarmin/RegressionTestProject/archive/refs/heads/4.0.zip I got this errors:

SCRIPT ERROR: Trying to return a previously freed instance.
          at: get_object (res://AutomaticBugs/ValueCreator.gd:289)

In master I don't have similar

[vnen]

@qarmin thanks, I've mistakenly inverted the logic in the get_validated_object_with_check() call (the boolean flag is for "freed" but I wrongly assumed it was for "valid").

[akien-mga]

Thanks!