Fix bug in OnSetsPerm

[james-d-mitchell]

Before this change, I get:

gap> InstallOtherMethod(POW, "for a character and perm",
> [IsChar, IsPerm],
> function(char, p)
>   return 1;
> end);
gap>
gap> OnSets("abc", (1,2));
[ 1, 131135, 1, 393219, <lvars bag> ]
gap>

Because the length of the output list is not properly set in the function OnSetsPerm in permutat.c. This PR fixes this issue. I haven't added a test because I wouldn't want to install the method for POW given above.