Fix PrimePGroup for direct products of trivial groups

[wilfwilson]

In PR #1722, I resolved issue #1719: there was a bug in an Assert in PrimePGroup for a direct product of groups.

I have discovered a further problem in PrimePGroup for a direct product of groups, which was present both before and after my earlier PR. In particular, PrimePGroup for trivial group should return fail, however for a direct product of trivial groups, PrimePGroup triggers an error. I don't think I fully appreciated before that a trivial group is a p-group in GAP.

This PR resolves the problem.

This is an example of the problem:

gap> G := DirectProduct(TrivialGroup(IsPcGroup), TrivialGroup(IsPcGroup));
<pc group of size 1 with 0 generators>
gap> PrimePGroup(G);
Error, no method found! For debugging hints type ?Recovery from NoMethodFound
The 1st argument is 'fail' which might point to an earlier problem
Error, no 1st choice method found for `IsPGroup' on 1 arguments at /Users/Wilf/GAP/lib/methsel2.g:241 called from
<compiled or corrupted statement>  called from
PrimePGroup( First( groups, function ( G )
        return PrimePGroup( G ) <> fail;
    end ) ) at /Users/Wilf/GAP/lib/gprd.gi:269 called from
<function "unknown">( <arguments> )

This is now fixed:

gap> G := DirectProduct(TrivialGroup(IsPcGroup), TrivialGroup(IsPcGroup));
<pc group of size 1 with 0 generators>
gap> PrimePGroup(G);
fail

[codecov]

Codecov Report

Merging #1754 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1754      +/-   ##
==========================================
+ Coverage   63.43%   63.44%   +<.01%     
==========================================
  Files        1000     1000              
  Lines      322760   322766       +6     
  Branches    13015    13038      +23     
==========================================
+ Hits       204757   204764       +7     
+ Misses     115361   115360       -1     
  Partials     2642     2642

Impacted Files	Coverage Δ
lib/gprd.gi	49.62% <100%> (+0.31%)	⬆️
src/funcs.c	70.86% <0%> (-0.29%)	⬇️
src/hpc/threadapi.c	34.17% <0%> (-0.29%)	⬇️
src/stats.c	73.36% <0%> (-0.14%)	⬇️
src/hpc/traverse.c	78.29% <0%> (+0.08%)	⬆️
src/lists.c	55.08% <0%> (+0.11%)	⬆️
src/hpc/thread.c	46.64% <0%> (+0.19%)	⬆️
src/objset.c	82.59% <0%> (+0.23%)	⬆️
lib/rwspcgrp.gi	74.49% <0%> (+1.01%)	⬆️

[markuspf]

Thanks for catching this. I think this is something I was trying to provoke when testing your last PR and then got distracted by the definition of IsPGroup. 👍