Bug#44040 MySQL allows creating a MERGE table upon VIEWs but crashes when

using it.
The crash was due to a null pointer present for select_lex while 
processing the view.
Adding a check while opening the view to see if its a child of a 
merge table fixed this problem.

mysql-test/r/merge.result:
  Updated result for the new test case.
mysql-test/t/merge.test:
  Added test case based on the bug description.
sql/sql_base.cc:
  Added a check to check if the view being opened is a child table of a
  merge table and return error if it is.

mysql-test/r/merge.result

@@ -2115,4 +2115,16 @@ insert into m1 (col1) values (1);
 insert into m1 (col1) values (1);
 ERROR 23000: Duplicate entry '' for key '*UNKNOWN*'
 drop table m1, t1;
+CREATE TABLE t1 (
+col1 INT(10)
+) ENGINE=MyISAM  DEFAULT CHARSET=latin1;
+CREATE VIEW v1 as SELECT * FROM t1;
+CREATE TABLE m1 (
+col1 INT(10)
+)ENGINE=MRG_MyISAM DEFAULT CHARSET=latin1 INSERT_METHOD=LAST UNION=(v1);
+#Select should detect that the child table is a view and fail.
+SELECT * FROM m1;
+ERROR HY000: Unable to open underlying table which is differently defined or of non-MyISAM type or doesn't exist
+DROP VIEW v1;
+DROP TABLE m1, t1;
 End of 5.1 tests

mysql-test/t/merge.test

@@ -1514,4 +1514,25 @@ insert into m1 (col1) values (1);
 insert into m1 (col1) values (1);
 
 drop table m1, t1;
+
+#
+#Bug #44040   MySQL allows creating a MERGE table upon VIEWs but crashes 
+#when using it
+#
+
+CREATE TABLE t1 (
+        col1 INT(10)
+) ENGINE=MyISAM  DEFAULT CHARSET=latin1;
+
+CREATE VIEW v1 as SELECT * FROM t1;
+CREATE TABLE m1 (
+        col1 INT(10)
+)ENGINE=MRG_MyISAM DEFAULT CHARSET=latin1 INSERT_METHOD=LAST UNION=(v1);
+
+--echo #Select should detect that the child table is a view and fail.
+--error ER_WRONG_MRG_TABLE
+SELECT * FROM m1;
+
+DROP VIEW v1;
+DROP TABLE m1, t1;
 --echo End of 5.1 tests

sql/sql_base.cc

@@ -3843,6 +3843,16 @@ retry:
 
   if (share->is_view)
   {
+    /*
+      If parent_l of the table_list is non null then a merge table
+      has this view as child table, which is not supported.
+    */
+    if (table_list->parent_l)
+    {
+      my_error(ER_WRONG_MRG_TABLE, MYF(0));
+      goto err;
+    }
+
     /*
       This table is a view. Validate its metadata version: in particular,
       that it was a view when the statement was prepared.