MDEV-35009: Initialize affected_rows in SQL service

The SQL service leaves the affected rows uninitialized.

The initialization of the spider plugin that uses
this service will fail under MSAN because there isn't
an initialized value to return at the end of the query.

Author: grooverdan

sql/sql_prepare.cc

@@ -6485,6 +6485,7 @@ extern "C" MYSQL *mysql_real_connect_local(MYSQL *mysql)
     new_thd->variables.wsrep_on= 0;
     new_thd->client_capabilities= client_flag;
     new_thd->variables.sql_log_bin= 0;
+    new_thd->affected_rows= 0;
     new_thd->set_binlog_bit();
     /*
       TOSO: decide if we should turn the auditing off