MPEG4Writer: Fix MediaServer crash in MM Stress test.

  - Media Server crash is happening while accessing
    codecspecific data at the end of recording since
    stopRecording is issued from script even before
    codec spefic data is returned from encoder.
  - Size check is added to prevent accessing
    codecspecific data in this case.

 CRs-Fixed: 732893

Change-Id: I77ce3c02c76c4d3cc4345d761ad505090ef29530