rtsp: fix integer overflow caused by malformed packets

Bug: 123940919
Test: adb shell am start -a android.intent.action.VIEW \
  -n com.google.android.apps.photos/.pager.HostPhotoPagerActivity \
  -t video/'*' -d rtsp://<rtsp_server2.py.host>/a.mp4
Change-Id: I2ef55d218e91aa4134150895ccf49ff81bee5891