MDEV-16980 Wrongly set tablename len while opening the

table for purge thread

Problem:
=======
	Purge tries to fetch mdl lock for the whole table even though it tries
to open one of the partition. But table name length was wrongly set to indicate
the partition name too.

Solution:
========
- Table name length should identify the table name only not the partition name.

sql/table.h

@@ -1765,6 +1765,8 @@ struct TABLE_LIST
       mdl_type= MDL_SHARED_READ;
 
     bzero((char*) this, sizeof(*this));
+    DBUG_ASSERT(!db_name_arg || strlen(db_name_arg) == db_length_arg);
+    DBUG_ASSERT(!table_name_arg || strlen(table_name_arg) == table_name_length_arg);
     db= (char*) db_name_arg;
     db_length= db_length_arg;
     table_name= (char*) table_name_arg;