Avoid a double decrease of reference counter for unstable provider

In some APIs unstable provider is released twice.
Unexpected release of unstable provider makes the reference counter
for unstable provider invalid state and it causes application crash.

This change avoids unexpected double decrease of reference counter
for unstable provider by setting null to prevent extra release
in finally block.

Change-Id: Ie2985c316ffcb6a9db7570c46347035c24d226af