`bench.xyz/1707_0569xWATER.xyz` can not be optimized with GFN-FF

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Describe the bug Running of GFN-FF optimization of bench.xyz/1707_0569xWATER.xyz fails with:

 * calculating model hessian...
Using GFN-FF Lindh-Hessian
 * fragmented diagonalization... 2 fragments
 Shifting diagonal of input Hessian by    2.0062370709201787E-002
 Lowest  eigenvalues of input Hessian
   -0.000062   -0.000024    0.000000    0.000000    0.000000    0.000000
    0.000000    0.000000    0.000000    0.000000    0.000000    0.000000
    0.000000    0.000000    0.000000    0.000000    0.000000    0.000000
 Highest eigenvalues
    0.455528    0.455639    1.353426    1.353680    1.461000    1.461173

 k=          18   nvar=        5121
 ANC generation failed

At the same time, geometry optimization with GFN2 starts:

 * calculating model hessian...
Using Lindh-Hessian (1995)
 Shifting diagonal of input Hessian by    1.6758195813745262E-002
 Lowest  eigenvalues of input Hessian
    0.010000    0.011217    0.011716    0.011716    0.012114    0.012630
    0.013063    0.013087    0.013463    0.013491    0.013663    0.014041
    0.014349    0.014389    0.014667    0.014703    0.014828    0.015191
 Highest eigenvalues
    1.360496    1.360685    1.363306    1.363780    1.363947    1.379580

To Reproduce Steps to reproduce the behaviour:

1. See #1258
2. GFN-FF: ../xtb --gfnff bench.xyz/1707_0569xWATER.xyz --opt --cycles 30 -P 1
3. GFN-2: ../xtb --gfn 2 bench.xyz/1707_0569xWATER.xyz --opt --cycles 30 -P 1

Expected behaviour

Start geometry optimization with GFN-FF

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See also #800