MDEV-17932 : assertion in multi RENAME USER command.

Sort acl_users inside the mysql_rename_user()'s loop, after every
successful iteration.

This is needed because on the next loop's iteration find_user_exact()
is used, which requires correct sorting by name.

mysql-test/main/grant.result

@@ -2762,3 +2762,11 @@ DROP USER dummy@localhost;
 #
 # End of 10.2 tests
 #
+#
+# MDEV-17932 : Assertion upon double RENAME USER
+#
+CREATE USER foo@localhost;
+CREATE USER bar2@localhost;
+RENAME USER foo@localhost TO bar1@localhost, bar1@localhost TO bar3@localhost;
+DROP USER bar2@localhost;
+DROP USER bar3@localhost;

mysql-test/main/grant.test

@@ -2258,3 +2258,12 @@ DROP USER dummy@localhost;
 --echo #
 --echo # End of 10.2 tests
 --echo #
+
+--echo #
+--echo # MDEV-17932 : Assertion upon double RENAME USER
+--echo #
+CREATE USER foo@localhost;
+CREATE USER bar2@localhost;
+RENAME USER foo@localhost TO bar1@localhost, bar1@localhost TO bar3@localhost;
+DROP USER bar2@localhost;
+DROP USER bar3@localhost;

sql/sql_acl.cc

@@ -10365,9 +10365,9 @@ bool mysql_rename_user(THD *thd, List <LEX_USER> &list)
       continue;
     }
     some_users_renamed= TRUE;
+    rebuild_acl_users();
   }
 
-  rebuild_acl_users();
 
   /* Rebuild 'acl_check_hosts' since 'acl_users' has been modified */
   rebuild_check_host();