bug in factoring out constant literal

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


```
sage: R1 = PolynomialRing(QQ, 'x,y,z')
sage: R1.0
Traceback (most recent call last):
...
NameError: name 'R1_sage_const_p0' is not defined
sage: R1.gen(0)
x
```

Component: **misc**

_Issue created by migration from https://trac.sagemath.org/ticket/5078_

