[Patch, Fortran] PR58436 - Fix a CLASS(*) finalization bug

[Tobias Burnus]

There were two related issues with the _vtab of CLASS(*): First, it was 
crashing in the generation of the finalization wrapper. Secondly, one 
needs to ensure that the _vtab contains a NULL pointer for _final.

Build and regtested on x86-64-gnu-linux.
OK for the trunk?
Tobias