MDEV-5047 virtual THD::~THD(): Assertion `status_var.memory_used == 0' fails

- Don't forget to delete the query plan footprint when 
  the query wasn't printed into slow query log for some reason
- ALso removed some garbage code.

mysql-test/r/explain_slowquerylog.result

@@ -9,3 +9,14 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 # 
 set autocommit=1;
 drop table t0;
+# 
+# MDEV-5047 virtual THD::~THD(): Assertion `status_var.memory_used == 0' fails on disconnect
+# 
+ALTER TABLE nonexisting ENABLE KEYS;
+ERROR 42S02: Table 'test.nonexisting' doesn't exist
+SHOW WARNINGS;
+Level	Code	Message
+Error	1146	Table 'test.nonexisting' doesn't exist
+SELECT 1;
+1
+1

mysql-test/t/explain_slowquerylog.test

@@ -16,5 +16,15 @@ explain select * from t0 where a < 3;
 --echo # MDEV-5045: Server crashes in QPF_query::print_explain with log_slow_verbosity='query_plan,explain'
 --echo # 
 set autocommit=1;
-
 drop table t0;
+
+--echo # 
+--echo # MDEV-5047 virtual THD::~THD(): Assertion `status_var.memory_used == 0' fails on disconnect
+--echo # 
+--connect (con1,localhost,root,,)
+--error ER_NO_SUCH_TABLE
+ALTER TABLE nonexisting ENABLE KEYS;
+SHOW WARNINGS;
+--disconnect con1
+--connection default
+SELECT 1;

sql/sql_parse.cc

@@ -1525,6 +1525,10 @@ bool dispatch_command(enum enum_server_command command, THD *thd,
 }
 
 
+/*
+  @note
+    This function must call delete_qpf_query().
+*/
 void log_slow_statement(THD *thd)
 {
   DBUG_ENTER("log_slow_statement");
@@ -1543,7 +1547,10 @@ void log_slow_statement(THD *thd)
   if (!thd->enable_slow_log ||
       (thd->variables.log_slow_filter
         && !(thd->variables.log_slow_filter & thd->query_plan_flags)))
+  {
+    delete_qpf_query(thd->lex);
     DBUG_VOID_RETURN; 
+  }
  
   if (((thd->server_status & SERVER_QUERY_WAS_SLOW) ||
        ((thd->server_status &
@@ -4799,13 +4806,7 @@ finish:
 #endif
   }
   lex->unit.cleanup();
-  //psergey-todo: print EXPLAIN here? After the above JOIN::cleanup calls?
+
-  // how do we print EXPLAIN extended, then?
-  if (lex->describe)
-  {
-    DBUG_ASSERT(lex->query_plan_footprint);
-    ///..
-  }
   /* Free tables */
   thd_proc_info(thd, "closing tables");
   close_thread_tables(thd);