Fixes a reported (but false) race condition in tsan for fate-apng:

WARNING: ThreadSanitizer: data race (pid=6274)
  Read of size 4 at 0x7d680001ec78 by main thread (mutexes: write M1338):
    #0 update_thread_context src/libavcodec/pngdec.c:1456 (ffmpeg+0x000000dacf0c)
[..]
  Previous write of size 4 at 0x7d680001ec78 by thread T1 (mutexes: write M1335):
    #0 decode_idat_chunk src/libavcodec/pngdec.c:737 (ffmpeg+0x000000dae951)