DB Connection pool

In mariadb_connection.c: MrdbConnection_close does not increment references to Py_None. This leads to de-reference a null object error after some number of get_connection/close connection of the pool.

Replacing (line 580)
pthread_mutex_unlock(&self->pool->lock);
return Py_None;

with
pthread_mutex_unlock(&self->pool->lock);
Py_INCREF(Py_None);
return Py_None;

fixes the problem.