MDEV-4824 userstats - wrong user statistics

(and valgrind warnings)

* move thd userstat initialization to the same function
  that was adding thd userstat to global counters.
* initialize thd->start_bytes_received in THD::init
  (when thd->userstat_running is set)

mysql-test/r/userstat-badlogin-4824.result

@@ -0,0 +1,16 @@
+create user foo@localhost identified by 'foo';
+flush user_statistics;
+set global userstat=1;
+select 1;
+1
+1
+select user, bytes_received from information_schema.user_statistics where user = 'foo';
+user	bytes_received
+foo	18
+connect(localhost,foo,bar,test,MASTER_PORT,MASTER_SOCKET);
+ERROR 28000: Access denied for user 'foo'@'localhost' (using password: YES)
+select user, bytes_received from information_schema.user_statistics where user = 'foo';
+user	bytes_received
+foo	18
+drop user foo@localhost;
+set global userstat=0;

mysql-test/t/userstat-badlogin-4824.test

@@ -0,0 +1,33 @@
+#
+# MDEV-4824 userstats - wrong user statistics
+#
+--source include/not_embedded.inc
+
+create user foo@localhost identified by 'foo';
+flush user_statistics;
+set global userstat=1;
+
+connect(foo, localhost, foo, foo);
+select 1;
+disconnect foo;
+connection default;
+
+# wait for user_statistics changes to become visible
+let $wait_condition= select count(*) = 1 from information_schema.processlist;
+--source include/wait_condition.inc
+
+# 41 is for ps-procotol
+--replace_result 41 18
+select user, bytes_received from information_schema.user_statistics where user = 'foo';
+
+--replace_result $MASTER_MYSOCK MASTER_SOCKET $MASTER_MYPORT MASTER_PORT
+--error ER_ACCESS_DENIED_ERROR
+connect(foo, localhost, foo, bar);
+
+connection default;
+
+--replace_result 41 18
+select user, bytes_received from information_schema.user_statistics where user = 'foo';
+
+drop user foo@localhost;
+set global userstat=0;