[C-API] Bugfix for ml_pipeline_element_get_handle

[C-API] Bugfix for ml_pipeline_element_get_handle

Need to ref element obtained by iterate_element() and unref it in cleanup_node() for common_elem

- common_elem by ml_pipeline_element_get_handle() need to unref in cleaunup_node() because g_hash_table_lookup assign new memory.
  but normal operation, unref is not necessary, so add ref in iterate_element()
- Now, the element's finalize function is called after ml_pipeline_element_get_handle()

Signed-off-by: hyunil park <hyunil46.park@samsung.com>