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

Change-Id: I0d745fc299a9f55fcbf0f9e14744d12198e96f42