mm-video: vidc: Missing release of pending input buffers to client

For unsupported contents with codec config buffer flag set, input
buffers in the pending queue are not released back to client
after FLUSH causing native crash. This change ensures that buffers
are released properly during flush in arbitrary mode

CRs-Fixed: 640296
Change-Id: I14b33c4a81d02d3b579de651c737fb531cb62dcc