Fix preimage map of residue field

[thofma]

Fixes #1284

[codecov]

Codecov Report

Merging #1285 (30833a3) into master (d65dbb3) will decrease coverage by 0.03%.
Report is 4 commits behind head on master.
The diff coverage is 81.81%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1285      +/-   ##
==========================================
- Coverage   74.58%   74.55%   -0.03%     
==========================================
  Files         346      346              
  Lines      111379   111383       +4     
==========================================
- Hits        83070    83046      -24     
- Misses      28309    28337      +28     

Files	Coverage Δ
src/Map/NfOrd.jl	66.75% <100.00%> (+0.08%)	⬆️
src/Misc/Integer.jl	49.33% <100.00%> (-0.29%)	⬇️
src/Misc/jordan.jl	58.04% <100.00%> (+0.22%)	⬆️
src/NumField/QQ.jl	53.12% <100.00%> (+1.92%)	⬆️
src/GrpAb/GrpAbFinGen.jl	80.39% <0.00%> (-0.10%)	⬇️
src/QuadForm/Quad/NormalForm.jl	99.55% <50.00%> (-0.15%)	⬇️

... and 26 files with indirect coverage changes