Fix an unitiliazed variable for the geometry optimization.
Description
Fix an unitiliazed variable for the geometry optimization. The comparisons are also made such that fixed_atom is supposed to larger than 0 instead of different from zero. This prevents that a negative value leads to segmentation fault.
News snippet
Fix an unitiliazed variable .
Checklist
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I have checked that my code follows the Octopus coding standards -
I have added tests for all the new features added in this request.