libstagefright: Checks before freeing buffers on a port

- OMXCodec calls dequeueBuffer on the NativeWindow in a loop
  and registers output buffers with the OMX component.
- If a dequeueBuffer fails, the buffers already registered
  with the OMX component were not getting freed laeding
  to a memory leak.
- On an error, check state of OMXCodec before freeing buffers on a port.

CRs-Fixed: 393352

(cherry picked from commit f2e982497c5d16db14812ee09cef5cbdf0d96331)

Change-Id: Ib4eb48c189197d253e4175708ce1778b9c92f315