fix cursor_cext rowcount error

[voidbao]

error demo:

import mysql.connector
conn = mysql.connector.connect(host='xxx', user='xxx', passwd='xxx', db='xxx')
cursor = conn.cursor()
cursor.execute('show tables')
print(cursor.rowcount)
print(cursor.fetchall()
print(cursor.rowcount)

the first cursor.rowcount will return 0, but account PEP 249: https://www.python.org/dev/peps/pep-0249/, it should be -1

the default c cursor is wrong, but the pure python cursor done it right, so i fix the c cursor base on the python cursor

[mysql-oca-bot]

Hi, thank you for submitting this pull request. In order to consider your code we need you to sign the Oracle Contribution Agreement (OCA). Please review the details and follow the instructions at https://oca.opensource.oracle.com/
Please make sure to include your MySQL bug system user (email) in the returned form.
Thanks

[voidbao]

Hi, thank you for submitting this pull request. In order to consider your code we need you to sign the Oracle Contribution Agreement (OCA). Please review the details and follow the instructions at https://oca.opensource.oracle.com/ Please make sure to include your MySQL bug system user (email) in the returned form. Thanks

have done

[mysql-oca-bot]

Hi, there was no response to our request to sign an OCA or confirm the code is submitted under the terms of the OCA. As such this request will be closed.
Thanks