save field_dict len validation.

Running this code:

>>> x = ModelInstance()
>>> x.save()  

Trigger this:

```
 1719     def execute_sql(self, sql, params=None, require_commit=True):
   1720         cursor = self.get_cursor()
-> 1721         res = cursor.execute(sql, params or ())
   1722         if require_commit and self.get_autocommit():
   1723             self.commit()

OperationalError: near ")": syntax error
```