ffmpeg: make transcode_init_done atomic.

Should fix tsan warnings in fate-fifo-muxer-h264/wav:

WARNING: ThreadSanitizer: data race (pid=26552)
  Write of size 4 at 0x000001e0d7c0 by main thread:
    #0 transcode_init src/ffmpeg.c:3761 (ffmpeg+0x00000050ca1c)
[..]
  Previous read of size 4 at 0x000001e0d7c0 by thread T1:
    #0 decode_interrupt_cb src/ffmpeg.c:460 (ffmpeg+0x0000004fde19)
