Bug in display2d for maxima interface

<div id="comment:0"></div>


```
   sage: maxima.de_solve('diff(w,x,2) + 2*diff(w,x)+2*w = 0', ['x','w'], [0,1,2])
w = %e^-x*(3*sin(x) + cos(x))
sage: maxima.de_solve('diff(w,x,2) + 2*diff(w,x)+2*w = 0', ['x','w'], [0,1,2]).display2d()
(output looks funny)
```



Component: **basic arithmetic**

_Issue created by migration from https://trac.sagemath.org/ticket/36_

