MDEV-7798: mysql.server init script can't stop mysqld when WSREP is turned off

wsrep threads should be always be checked for existence & stopped on
server shutdown.

mysql-test/suite/wsrep/r/mdev_7798.result

+#
+# MDEV-7798: mysql.server init script can't stop mysqld when WSREP is
+# turned off
+#
+SELECT @@GLOBAL.WSREP_ON;
+@@GLOBAL.WSREP_ON
+1
+SET GLOBAL WSREP_ON= 0;
+Restart the node.
+SELECT @@GLOBAL.WSREP_ON;
+@@GLOBAL.WSREP_ON
+1
+# End of test.

mysql-test/suite/wsrep/t/mdev_7798.opt

+--wsrep-provider=$WSREP_PROVIDER --wsrep-cluster-address=gcomm:// --wsrep-on=1 --wsrep_causal_reads=ON

mysql-test/suite/wsrep/t/mdev_7798.test

+--source include/have_wsrep_provider.inc
+--source include/have_binlog_format_row.inc
+
+--echo #
+--echo # MDEV-7798: mysql.server init script can't stop mysqld when WSREP is
+--echo # turned off
+--echo #
+
+SELECT @@GLOBAL.WSREP_ON;
+SET GLOBAL WSREP_ON= 0;
+
+--echo Restart the node.
+--source include/restart_mysqld.inc
+
+SELECT @@GLOBAL.WSREP_ON;
+
+--echo # End of test.

sql/mysqld.cc

@@ -1753,7 +1753,8 @@ static void __cdecl kill_server(int sig_ptr)
   }
 #endif
 #ifdef WITH_WSREP
-  if (WSREP_ON) wsrep_stop_replication(NULL);
+  /* Stop wsrep threads in case they are running. */
+  wsrep_stop_replication(NULL);
 #endif
 
   close_connections();