frame_thread_encoder: make 'exit' member atomic.

Should fix the following tsan warning:

WARNING: ThreadSanitizer: data race (pid=19806)
  Read of size 4 at 0x7b84000012f0 by thread T9:
    #0 worker src/libavcodec/frame_thread_encoder.c:66 (ffmpeg+0x0000007f349e)
[..]
  Previous write of size 4 at 0x7b84000012f0 by main thread (mutexes: write M1395):
    #0 ff_frame_thread_encoder_free src/libavcodec/frame_thread_encoder.c:239 (ffmpeg+0x0000007f379e)
[..]
