PowerSeries_poly.reverse() should be more careful with conversions to pari

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


```
sage: Series = PowerSeriesRing(SR, 'x')
sage: ser = Series([0, pi])
sage: ser
pi*x
sage: ser.reverse()
0.318309886183791*x + O(x^20)
```

Component: **algebra**

_Issue created by migration from https://trac.sagemath.org/ticket/20847_

