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 Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• 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).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
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[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 80.49%. Comparing base (288eb04) to head (1f1968b).

Files with missing lines	Patch %	Lines
src/solvers/floatbv/float_utils.cpp	75.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8968   +/-   ##
========================================
  Coverage    80.49%   80.49%           
========================================
  Files         1704     1704           
  Lines       188804   188808    +4     
  Branches        73       73           
========================================
+ Hits        151982   151988    +6     
+ Misses       36822    36820    -2     

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