Fix ref counting bug in AHardwareBuffer_recvHandleFromUnixSocket

Follow-on from I443015d63245e49e8cf38847030c9da8142cbe50. If unflatten
returns an error, this ensures that the GraphicBuffer is deleted. This
matches similar code in this file.
Bug: NA
Test: make
Test: AHardwareBufferTest_SendAndRecvSucceeds

Change-Id: Ibcf21347625e3c9df127a0267aa05b046132e0a0