float_utils: normalization shift for very short fractions

[kroening]

Instead of erroring, this uses zero extension when the fraction given to the normalization shift is shorter than the distances considered.

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• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
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• My PR is restricted to a single feature or bugfix.
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[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 80.50%. Comparing base (453c863) to head (425cbfa).
⚠️ Report is 4 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/solvers/floatbv/float_utils.cpp	66.66%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8968      +/-   ##
===========================================
- Coverage    80.50%   80.50%   -0.01%     
===========================================
  Files         1704     1704              
  Lines       188803   188806       +3     
  Branches        73       73              
===========================================
+ Hits        152004   152005       +1     
- Misses       36799    36801       +2     

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