Bug#41305 server crashes when inserting duplicate row into a merge table

This problem comes while inserting a duplicate row in merge table 
without key but the child table has a primary key. 
While forming the error message handler tries to locate the key field
which is creating this problem but as there is no key on the merge
table there is a segmentation fault.

mysql-test/r/merge.result

@@ -2103,4 +2103,16 @@ a
 UNLOCK TABLES;
 # drop the created tables
 DROP TABLE t1, t2, t3;
+# insert duplicate value in child table while merge table doesn't have key
+create table t1 (
+col1 int(10),
+primary key (col1)
+) ENGINE=MyISAM DEFAULT CHARSET=latin1;
+CREATE TABLE m1 (
+col1 int(10) NOT NULL
+) ENGINE=MRG_MyISAM DEFAULT CHARSET=latin1 INSERT_METHOD=LAST UNION=(t1);
+insert into m1 (col1) values (1);
+insert into m1 (col1) values (1);
+ERROR 23000: Duplicate entry '' for key '*UNKNOWN*'
+drop table m1, t1;
 End of 5.1 tests

mysql-test/t/merge.test

@@ -1496,4 +1496,22 @@ UNLOCK TABLES;
 --echo # drop the created tables
 DROP TABLE t1, t2, t3;
 
+#
+# Bug #41305 server crashes when inserting duplicate row into a merge table
+#
+--echo # insert duplicate value in child table while merge table doesn't have key
+create table t1 (
+        col1 int(10),
+        primary key (col1)
+) ENGINE=MyISAM DEFAULT CHARSET=latin1;
+
+CREATE TABLE m1 (
+  col1 int(10) NOT NULL
+) ENGINE=MRG_MyISAM DEFAULT CHARSET=latin1 INSERT_METHOD=LAST UNION=(t1);
+
+insert into m1 (col1) values (1);
+--error ER_DUP_ENTRY
+insert into m1 (col1) values (1);
+
+drop table m1, t1;
 --echo End of 5.1 tests

storage/myisammrg/ha_myisammrg.cc

@@ -872,6 +872,16 @@ int ha_myisammrg::info(uint flag)
     table->s->crashed= 1;
 #endif
   stats.data_file_length= mrg_info.data_file_length;
+  if (mrg_info.errkey >= table_share->keys) 
+  {
+    /*
+     If value of errkey is higher than the number of keys
+     on the table set errkey to MAX_KEY. This will be
+     treated as unknown key case and error message generator
+     won't try to locate key causing segmentation fault.
+    */
+    mrg_info.errkey= MAX_KEY;
+  }
   errkey= mrg_info.errkey;
   table->s->keys_in_use.set_prefix(table->s->keys);
   stats.mean_rec_length= mrg_info.reclength;