BUG: xs with drop_level=True with a fully specified tuples buggy
related #6501

```
In [1]: n=23

In [2]: df = pd.DataFrame({'a':np.append(np.random.randint(0,10,n), -1),
   ...:                    'b':np.append(np.random.randint(0,10,n), -1),
   ...:                    'c':np.append(np.random.randint(0,10,n), -1),
   ...:                    'value':np.random.randint(0,100,n+1)})
In [3]: df.set_index(['a','b','c'], inplace=True)

In [4]: df.sortlevel(inplace = True)

In [7]: df.xs((-1,-1,-1),drop_level=True)
Out[7]: 
value    40
Name: (-1, -1, -1), dtype: int64

In [8]: df.xs((-1,-1),drop_level=True)
Out[8]: 
    value
c        
-1     40

[1 rows x 1 columns]
```
