cbind/rbind fail on matrixes containing lists (PR#6702)

Yep, bind.c:do_bind has

    switch(mode) {
    case NILSXP:
    case LGLSXP:
    case INTSXP:
    case REALSXP:
    case CPLXSXP:
    case STRSXP:
        break;
    default:
        errorcall(call, "cannot create a matrix from these types");
    }


Looks like a fairly simple fix to me.

	-thomas