Guard usage of RData for already freed Data objects

When a Data object is already freed, we can't use the raw pointer
anymore that it used to point to some C allocated data. When this
happens we raise an exception so code after for example a call to
DATA_PTR, can't run and cause segfaults.
Reenable Ruby finalizers

With the additional guard for Data objects and native memory, this can't
cause crashes anymore now.
Reenable finalizers specs