Fix duplicated buffer issue for output buffer

Fix duplicated buffer issue for output buffer

- Previously, the mutex for output buffer was locked
  after return rpc_port_proxy_codec_invoke_release_output_buffer().
  The problem is that output_buffer[] array could be accessed in message_cb()
  before remove released buffer in output_buffer[] array
  and it caused duplicated output buffer error although it's released actually.

[Version] 1.5.1
[Issue Type] Bug fix

Change-Id: I31ac0afba1ae5323ab19c52cf3a41862ec54543f
Signed-off-by: Jeongmo Yang <jm80.yang@samsung.com>