MTX.IsAbsolutelyIrreducible() sometimes fails for modules created from matrix objects. #6670

gap> gens := List( GeneratorsOfGroup( Omega( 1, 4, 8 ) ), x -> Matrix( IsPlistMatrixRep, GF(8), x ) );
[ <4x4-matrix over GF(2^3)>, <4x4-matrix over GF(2^3)> ]
gap> Reset( GlobalMersenneTwister, 2 );;
gap> Reset( GlobalRandomSource, 2 );;
gap> M := GModuleByMats( gens, GF(8) );;
gap> M;
rec( IsOverFiniteField := true, dimension := 4, field := GF(2^3),
  generators := [ <immutable 4x4-matrix over GF(2^3)>, <immutable 4x4-matrix over GF(2^3)> ], isMTXModule := true )
gap> MTX.IsAbsolutelyIrreducible( M );
Error, <mat> * <matobj> is not defined
Stack trace:
*[1] Error( "<mat> * <matobj> is not defined" );
   @ /home/peter/gap/gap/lib/matobj.gi:1240
 [2] P * matrices[i]
   @ /home/peter/gap/gap/lib/meataxe.gi:1764
 [3] SMTX.AbsoluteIrreducibilityTest( module )
   @ /home/peter/gap/gap/lib/meataxe.gi:294
<function "unknown">( <arguments> )
 called from read-eval loop at *stdin*:10
you can enter 'quit;' to quit to outer loop, or
you can enter 'return;' to continue