another preparser edge case

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


```
sage: 3.xgcd(0)
(3, 1, 0)
sage: 3._xgcd(0)
------------------------------------------------------------
   File "<ipython console>", line 1
     RealNumber('3.')_xgcd(Integer(0))
                         ^
<type 'exceptions.SyntaxError'>: invalid syntax
```


Component: **misc**

_Issue created by migration from https://trac.sagemath.org/ticket/1599_

