NULL out pointers after freeing them as defensive programming

In mariadb_lib.c, the mysql_close function contains this code:

if (mysql->net.extension)
free(mysql->net.extension);

mysql->host_info=mysql->user=mysql->passwd=mysql->db=0;

/* Clear pointers for better safety */
memset((char*) &mysql->options, 0, sizeof(mysql->options));

if (mysql->extension)
free(mysql->extension);

mysql->net.pvio= 0;
if (mysql->free_me)
free(mysql);

I recommend setting mysql->extension and mysql->net.extension to NULL also as a defensive programming technique. (This is already done for mysql->options.extension in the memset() call).

This refers to https://jira.mariadb.org/browse/CONC-289 and also fixes the DBD::mysql issue reported at https://github.com/perl5-dbi/DBD-mysql/issues/306

A patch is attached.

Regards,

Dianne.