Conversion check: fix off-by-one error for float-to-unsigned

[tautschnig]

We use less-than in the comparison, so make the bound one larger than the largest representable value (as we do in the other cases of float-to-signed/unsigned conversion checks).

Fixes: #8131

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[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (69bb2b6) 79.09% compared to head (285893a) 79.09%.

Files	Patch %	Lines
src/ansi-c/goto_check_c.cpp	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8157      +/-   ##
===========================================
- Coverage    79.09%   79.09%   -0.01%     
===========================================
  Files         1695     1695              
  Lines       196583   196583              
===========================================
- Hits        155496   155493       -3     
- Misses       41087    41090       +3     

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