lpsink: decrease the ref count of a/v sink when state has changed to NULL
lpsink: decrease the ref count of a/v sink when state has changed to NULL

Change-Id: I26e5a3e1cc29dac707898c7a1f6abed6d9e0af8e
Reviewed-on: https://gpro.palm.com/5399
Tested-by: HoonHee Lee <41497318ef67053f37c2ac0e974a3369f66d66ec@lge.com>
Reviewed-by: Jeongseok Kim <0c03ab7a1b32f29697716389c3f9549845834271@lge.com>

justinjoy/gst-plugins-lp,justinjoy/gst-plugins-lp