MDEV-15620 Crash when using "SET @@NEW.a=expr" inside a trigger

A simple patch fixing the problem in 5.5.
Note, a full patch was previously fixed to 10.3.

mysql-test/r/parser.result

@@ -672,3 +672,10 @@ ERROR 42000: You have an error in your SQL syntax; check the manual that corresp
 PREPARE stmt FROM 'CREATE TRIGGER tr AFTER DELETE ON t1 FOR EACH ROW SET @a = 1\\';
 ERROR 42000: You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near '\' at line 1
 DROP TABLE t1;
+#
+# MDEV-15620 Crash when using "SET @@NEW.a=expr" inside a trigger
+#
+CREATE TABLE t1 (a INT);
+CREATE TRIGGER tr1 BEFORE INSERT ON t1 FOR EACH ROW SET @@NEW.a=0;
+ERROR HY000: Unknown system variable 'NEW'
+DROP TABLE t1;

mysql-test/t/parser.test

@@ -780,3 +780,12 @@ CREATE TRIGGER tr AFTER DELETE ON t1 FOR EACH ROW SET @a = 1\;
 --error ER_PARSE_ERROR
 PREPARE stmt FROM 'CREATE TRIGGER tr AFTER DELETE ON t1 FOR EACH ROW SET @a = 1\\';
 DROP TABLE t1;
+
+--echo #
+--echo # MDEV-15620 Crash when using "SET @@NEW.a=expr" inside a trigger
+--echo #
+
+CREATE TABLE t1 (a INT);
+--error ER_UNKNOWN_SYSTEM_VARIABLE
+CREATE TRIGGER tr1 BEFORE INSERT ON t1 FOR EACH ROW SET @@NEW.a=0;
+DROP TABLE t1;

sql/sql_yacc.yy

@@ -13567,6 +13567,11 @@ option_value:
         | '@' '@' opt_var_ident_type internal_variable_name equal set_expr_or_default
           {
             struct sys_var_with_base tmp= $4;
+            if (tmp.var == trg_new_row_fake_var)
+            {
+              my_error(ER_UNKNOWN_SYSTEM_VARIABLE, MYF(0), "NEW");
+              MYSQL_YYABORT;
+            }
             /* Lookup if necessary: must be a system variable. */
             if (tmp.var == NULL)
             {