Fixes tsan warnings in fate-apng:

WARNING: ThreadSanitizer: data race (pid=51230)
  Read of size 4 at 0x7d50000042fc by main thread (mutexes: write M1000):
    #0 frame_copy_props frame.c:302 (ffmpeg:x86_64+0x1019a35d6)
[..]
  Previous write of size 4 at 0x7d50000042fc by thread T1 (mutexes: write M997):
    #0 decode_idat_chunk pngdec.c:708 (ffmpeg:x86_64+0x100f5562a)