Fix memory leaks when plugin gets no results

The mysql plugin currently leaks memory when it's not correctly setup, which isn't desirable. Whenever you log either of these type of messages:

Feb 26 12:29:03 jslave19 collectd[24575]: mysql plugin: Failed to get master statistics: `SHOW MASTER STATUS' did not return any rows.
Feb 26 12:29:03 jslave19 collectd[24575]: mysql plugin: Failed to get slave statistics: `SHOW SLAVE STATUS' did not return any rows.

then a chunk of RAM for the MySQL result gets leaked, as per valgrind trace below
==24575== 92,840 (2,024 direct, 90,816 indirect) bytes in 11 blocks are definitely lost in loss record 747 of 752
==24575==    at 0x4C274A8: malloc (vg_replace_malloc.c:236)
==24575==    by 0x8CE15B1: my_malloc (in /usr/lib/libmysqlclient_r.so.16.0.0)
==24575==    by 0x8D0C1BC: mysql_store_result (in /usr/lib/libmysqlclient_r.so.16.0.0)
==24575==    by 0x8A8B2BA: exec_query (mysql.c:374)
==24575==    by 0x8A8C2C1: mysql_read (mysql.c:397)
==24575==    by 0x40F3F1: plugin_read_thread (plugin.c:450)
==24575==    by 0x52BE9C9: start_thread (pthread_create.c:300)
==24575==    by 0xB4E06FF: ???

This patch should fix that, freeing that memory up.