MDEV-18748 REPLACE doesn't work with unique blobs on MyISAM table

on long unique conflict, set table->file->dup_ref for
engines that support it

mysql-test/main/long_unique_bugs.result

@@ -53,3 +53,7 @@ create temporary table t1 (f blob, unique(f)) engine=innodb;
 insert into t1 values (1);
 replace into t1 values (1);
 drop table t1;
+create table t (b blob, unique(b)) engine=myisam;
+insert into t values ('foo');
+replace into t values ('foo');
+drop table t;

mysql-test/main/long_unique_bugs.test

@@ -64,3 +64,11 @@ create temporary table t1 (f blob, unique(f)) engine=innodb;
 insert into t1 values (1);
 replace into t1 values (1);
 drop table t1;
+
+#
+# MDEV-18748 REPLACE doesn't work with unique blobs on MyISAM table
+#
+create table t (b blob, unique(b)) engine=myisam;
+insert into t values ('foo');
+replace into t values ('foo');
+drop table t;

sql/handler.cc

@@ -6558,7 +6558,14 @@ static int check_duplicate_long_entry_key(TABLE *table, handler *h,
     error= HA_ERR_LOCK_WAIT_TIMEOUT;
 exit:
   if (error)
+  {
     table->file->errkey= key_no;
+    if (h->ha_table_flags() & HA_DUPLICATE_POS)
+    {
+      h->position(table->check_unique_buf);
+      memcpy(table->file->dup_ref, h->ref, h->ref_length);
+    }
+  }
   h->ha_index_end();
   return error;
 }