IsMonimialMatrix does not work for compressed matrices

[DominikBernhardt]

When applying IsMonomialMatrix to a matrix from SL(n,q), GAP throws the error

Error, Panic: cannot convert <list> (is a object (positional)) to a plain list in
found := BlistList( M, [  ] ); at /usr/lib/gap/lib/matrix.gi:2580 called from
<function "last">( <arguments> )

Converting the entries of a matrix from SL(n,q) via IntFFE and then calling IsMonomialMatrix works
fine.

As @ssiccha pointed out, one can reproduce the Error via

gap> mm := [[0,1,0], [1,0,0]] * Z(3);;
gap> ConvertToMatrixRep(mm);
3
gap> BlistList(mm, []);
Error, Panic: cannot convert <list> (is a object (positional)) to a plain list
not in any function at *stdin*:26`

[fingolfin]

Note that this is an ancient bug, not a regression in GAP 4.10.0