This is a follow-up to 3191948625, which fixed
compile-time compatibility with newer ffmpeg, but introduced runtime crashes.
* gnu/packages/games.scm (stepmania): Replace ffmpeg-6 with ffmpeg.
* gnu/packages/patches/stepmania-ffmpeg-compat.patch: Add hunks for FFMPEG 8
compatibility.
Initialize m_pStreamCodec to nullptr and reinitialize it in
MovieDecoder_FFMpeg::OpenCodec.