problem with generic code for elements in pc groups

[ThomasBreuer]

The following happens in GAP 4.12.2 as well as in the current master branch.

gap> p:= NextPrimeInt( 134217688 );
134217689
gap> G:= CyclicGroup( p );
<pc group of size 134217689 with 1 generator>
gap> ConjugacyClassesSubgroups( G );;   # works
gap> p:= NextPrimeInt( p );
134217757
gap> G:= CyclicGroup( p );
<pc group of size 134217757 with 1 generator>
gap> ConjugacyClassesSubgroups( G );;   # does not work
Error, PosObj Element: <PosObj>![9] must have an assigned value in
  return Type![9]( Type![5], vec )
[...]

The problem is that different collectors are used for the two groups in question.
The smaller one still uses a 32BitsSingleCollector whereas the bigger one uses the generic PolycyclicCollector.
The element types for the collectors are created in MultiplicativeElementsWithInversesFamilyByRws methods.
The methods for the 8, 16, and 32 bits collectors set the entries at the relevant positions in these types, but this is not the case for the generic collector.