CCodec: Guard against null output->buffers

In CCodecBufferChannel::feedInputBufferIfAvailable(), there is a call to
output->buffers->hasPending(). Since we have changed the logic to allow
output->buffers to be null, the call to hasPending() must be done after
confirming that output->buffers is not null.

Test: atest CtsMediaTestCases

Bug: 158509743
Bug: 158539113
Change-Id: Icb1a46c954ff2b8d5018c0827ff9ca9acd8d4d8e