AMPEG4ElementaryAssembler: reject malformed desc

Bug: 124777537
Test: (1) python rtsp_server2.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: Iacc6882819e0d99ffdab409324301e8f8d8125f7