Datetime.datetime conversion fixed.

By switching from pyodbc I found this bug for a date string ('2009-05-02 00:00'):
```
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File ".\formatum\oe2010\dbisam.py", line 169, in verseny_adatok
    for é in kurzor.execute(sql_lek).fetchone())
  File ".\formatum\oe2010\pypyodbc.py", line 1861, in fetchone
    value_list.append(buf_cvt_func(alloc_buffer.value))
  File ".\formatum\oe2010\pypyodbc.py", line 570, in dttm_cvt
    return datetime.datetime(int(x[0:4]),int(x[5:7]),int(x[8:10]),int(x[10:13]),int(x[14:16]),int(x[17:19]),int(x[20:].ljust(6,'0')))
ValueError: invalid literal for int() with base 10: ''
```