CCBC: Fix null pointer exception at CCBC::flush()

The description of C2Work says:
"When the work is returned (via onWorkDone), the input buffer-pack's
buffer vector shall contain nullptrs." This CL handles the case that
C2Work's input buffers contains nullptr.

Bug: 174432542
Test: android.mediav2.cts.CodecDecoderSurfaceTest
Change-Id: Ife7fb849c88c82f92a0bb687b5f1f6bcb171686b