Walrus causes unnecessary SyntaxError

This is probably simple to fix, see:

```
>>> import jedi
>>> x = jedi.Script('f(a := 1, b)\n').get_syntax_errors()
>>> x[0].get_message()
'SyntaxError: positional argument follows keyword argument'
>>> jedi.Script('f(a := 1)\n').get_syntax_errors()
[]
```
