Fixes the following tsan warning when running fate-vsynth_lena-ffvhuff:

WARNING: ThreadSanitizer: data race (pid=6484)
  Write of size 8 at 0x7d64000154b8 by main thread (mutexes: write M1331):
    #0 update_context_from_user src/libavcodec/pthread_frame.c:331 (ffmpeg+0x000000dca887)
[..]
  Previous read of size 8 at 0x7d64000154b8 by thread T2 (mutexes: write M1334):
    #0 draw_slice src/libavcodec/huffyuvdec.c:857 (ffmpeg+0x000000bcc86f)