Security update for ffmpeg-4 (low)

This update for ffmpeg-4 to version 4.0.2 fixes the following issues:

These security issues were fixed:

• CVE-2018-15822: The flv_write_packet function did not check for an empty
  audio packet, leading to an assertion failure and DoS (bsc#1105869).
• CVE-2018-13300: An improper argument passed to the avpriv_request_sample
  function may have triggered an out-of-array read while converting a
  crafted AVI file to MPEG4, leading to a denial of service and possibly
  an information disclosure (bsc#1100348).

These non-security issues were fixed:

• Enable webvtt encoders and decoders (boo#1092241).
• Build codec2 encoder and decoder, add libcodec2 to enable_decoders and
  enable_encoders.
• Enable mpeg 1 and 2 encoders.