vidc: Flush the cached buffer only if the buffer is valid.

Before flushing the cached buffer check for the following
- When buffer allocation fails, do not flush the buffer
  report error and return.
- Check if the buffer handle is valid, before flushing the
  buffer.

CRs-Fixed: 358873
Change-Id: I58b869d2d2eb72f991649d62a5186c7b0f5adcf4
Signed-off-by: Deepak Kotur <dkotur@codeaurora.org>