MDEV-11162 Assertion `num_records == m_idx_array.size()' failed in Filesort_buffer::alloc_sort_buffer(uint, uint)

When JOIN::destroy() is called for a JOIN object that has
   - join->tmp_join != NULL
   - also has join->table[0]->sort

then the latter was not cleaned up.
This could cause a memory leak and/or asserts in the subsequent queries.

Fixed by adding a cleanup call.

sql/sql_select.cc

@@ -3144,6 +3144,7 @@ JOIN::destroy()
     */
     tmp_table_param.cleanup();
     tmp_join->tmp_table_param.copy_field= 0;
+    cleanup(1);
     DBUG_RETURN(tmp_join->destroy());
   }
   cond_equal= 0;