Incorrect behaviour of SET STATEMENT in case it is executed as a prepared statement

While working on the task MDEV-16708 (Unsupported commands for prepared statements)
I hit the bug caused by incorrect handling of the ' SET STATEMENT' clause when the whole statement, containing the 'SET STATEMENT' clause, is executed in prepared statement mode.

The following test case shows the incorrect behavior:

PREPARE stmt FROM "SET STATEMENT sql_mode = 'NO_ENGINE_SUBSTITUTION' FOR CREATE TABLE t1 AS SELECT CONCAT('abc') AS c1";

EXECUTE stmt;

DEALLOCATE PREPARE stmt;

SHOW CREATE TABLE t1;

Table	Create Table

t1	CREATE TABLE `t1` (

  `c1` varchar(3) DEFAULT NULL

) ENGINE=MyISAM DEFAULT CHARSET=latin1

DROP TABLE t1;

It's expected that the output from the command SHOW CREATE TABLE t1 shows
`c1` varchar(3) CHARACTER SET utf8 NOT NULL
for the column c1 as it takes place in case this statement is run as a regular statement.

Instead, SHOW CREATE TABLE t1 outputs the following data
`c1` varchar(3) CHARACTER SET utf8 DEFAULT NULL
as a definition of the column c1