stream_out/transcode: fix memory-leak on transcode_video_encoder_open failure
stream_out/transcode: fix memory-leak on transcode_video_encoder_open failure

fixes #17426

Signed-off-by: Thomas Guillem <5f50a84c1fa3bcff146405017f36aec1a10a9e38@gllm.fr>

xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc