Infinite loop in conversion of p-adics

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

Here is the bug:

```
sage: A.<a> = Qq(5^2)
sage: a in A.base_ring()
Traceback (most recent call last):
...
RecursionError: maximum recursion depth exceeded in comparison
```


CC:  @roed314

Component: **padics**

_Issue created by migration from https://trac.sagemath.org/ticket/28555_

