Fix an issue in TrySolvableSubgroup affecting anupq

[fingolfin]

This resolves gap-packages/anupq#67 where the following code would get stuck in the last line:

LoadPackage("anupq");;
G:=SmallGroup(512,383371);;
H:=SmallGroup(512,383419);;
PqStandardPresentation(G);
PqStandardPresentation(H);

See gap-packages/anupq#67 (comment) where @beick described this fix which was suggested by @hulpke

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.65%. Comparing base (6083a21) to head (4c638bd).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #13   +/-   ##
=======================================
  Coverage   88.65%   88.65%           
=======================================
  Files          13       13           
  Lines        1745     1746    +1     
=======================================
+ Hits         1547     1548    +1     
  Misses        198      198           

Files with missing lines	Coverage Δ
gap/nicestab.gi	74.41% <100.00%> (+0.19%)	⬆️

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[hulpke]

Programming logic is certainly fine. It would be nice to find out why phom cannot compute pre-images, but the inverse can compute images.
I know that I suggested the change, but it is admittedly in the "hack to make it work" category, and I did not fuillyu understand the underlying problem.

[fingolfin]

Thanks @hulpke that's helpful. I was also really puzzled by this but I didn't want to forget about it