MDEV-7222: Cluster Node Crash at CREATE DEFINER statement

Check whether the definer host string is not null before
appending it to the specified buffer.

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

+#
+# MDEV-7222: Cluster Node Crash at CREATE DEFINER statement
+#
+USE test;
+CREATE DEFINER=CURRENT_USER VIEW v1 AS SELECT 1;
+DROP VIEW v1;
+# End of tests

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

+--source include/galera_cluster.inc
+--source include/have_innodb.inc
+
+--echo #
+--echo # MDEV-7222: Cluster Node Crash at CREATE DEFINER statement
+--echo #
+USE test;
+CREATE DEFINER=CURRENT_USER VIEW v1 AS SELECT 1;
+DROP VIEW v1;
+
+--echo # End of tests

sql/sql_show.cc

@@ -2191,7 +2191,7 @@ void append_definer(THD *thd, String *buffer, const LEX_STRING *definer_user,
 {
   buffer->append(STRING_WITH_LEN("DEFINER="));
   append_identifier(thd, buffer, definer_user->str, definer_user->length);
-  if (definer_host->str[0])
+  if (definer_host->str && definer_host->str[0])
   {
     buffer->append('@');
     append_identifier(thd, buffer, definer_host->str, definer_host->length);