Describe the bug
Attempting to run different types of Hessian calculations (--hess, --bhess, --ohess) in combination with GFN-FF leads to crashes, even with an optimized starting geometry.

To Reproduce
Steps to reproduce the behavior:

1. happens with input file c2h4.xyz in the attached archive hess.zip
2. start xtb (binary release 6.7.0 from GitHub) with any of the following options:

   xtb c2h4.xyz --hess --gfnff --verbose >stdout 2>stderr
   xtb c2h4.xyz --bhess --gfnff --verbose >stdout 2>stderr
   xtb c2h4.xyz --ohess --gfnff --verbose >stdout 2>stderr
   

3. output showing the failed calculations for all three variants in the archive attached

Expected behaviour
Successful Hessian calculations with GFN-FF.

Additional context
These problems do not appear to happen without the --gfnff flag (that is with GFN2-xTB).