NuPlayerRenderer: Handle AMessage post error in openAudioSink

AMessage::postAndAwaitResponse can fail if the target looper no
longer exists or if allocation of the reply token fails. Check for
error in these cases to avoid deferencing a NULL response.
Test: compiles
Bug: 34805421

(cherry picked from commit b90c8f42036ea773e938cadd0d52480ac703edd7)

Change-Id: Ib76c111a39462e867a41f320c0ba0a220a0c5641