Fix overflow bug in C2xC2 field computation

[thofma]

No description provided.

[thofma]

Bug appears with bound ~10^12, so a bit hard to add a test.

[codecov]

Codecov Report

Patch coverage: 66.66% and project coverage change: +0.16 🎉

Comparison is base (de95733) 69.29% compared to head (4af83b0) 69.46%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1148      +/-   ##
==========================================
+ Coverage   69.29%   69.46%   +0.16%     
==========================================
  Files         349      349              
  Lines      113013   113018       +5     
==========================================
+ Hits        78318    78510     +192     
+ Misses      34695    34508     -187     

Impacted Files	Coverage Δ
src/FieldFactory/ab_exts.jl	57.15% <66.66%> (-0.23%)	⬇️

... and 46 files with indirect coverage changes

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