MXS-3408 Fix leak in qc_mysqlembedded properly

The actual cause was a missing cleanup call.

query_classifier/qc_mysqlembedded/qc_mysqlembedded.cc

@@ -1960,6 +1960,7 @@ static void parsing_info_done(void* ptr)
             {
                 thd = (THD*) mysql->thd;
                 thd->end_statement();
+                thd->cleanup_after_query();
                 (*mysql->methods->free_embedded_thd)(mysql);
                 mysql->thd = NULL;
             }

query_classifier/test/join.test

@@ -700,9 +700,7 @@ create table t2 (a int, b int, primary key(a));
 
 explain select * from t1;
 show status like '%cost%';
-#MXS The following line causes a leak in qc_mysqlembedded that causes ASAN to
-#MXS complain. Easiest to solve by commenting out.
-#select 'The cost of accessing t1 (dont care if it changes' '^';
+select 'The cost of accessing t1 (dont care if it changes' '^';
 
 select 'vv: Following query must use ALL(t1), eq_ref(A), eq_ref(B): vv' Z;