AMPEG4ElementaryAssembler: erase malformed packets

Bug: 124783982
Test: (1) python rtsp_serverTS.py $server_addr
Test: (2) adb shell am start -a android.intent.action.VIEW
  -n com.google.android.apps.photos/.pager.HostPhotoPagerActivity
  -t video/'*' -d rtsp://$server_addr/a.mp4
Change-Id: I7735eaa61ca4fd0d248a71d9604aae7f1c2e725f