Otherwise the thread may still be in the middle of decoding a previous
frame, which would effectively trigger a race condition on any field
concurrently read and written.

In practice, this fixes tsan warnings like the following:

WARNING: ThreadSanitizer: data race (pid=17380)
  Write of size 4 at 0x7d64000160fc by main thread:
    #0 update_context_from_user src/libavcodec/pthread_frame.c:335 (ffmpeg+0x000000dca515)
[..]
  Previous read of size 4 at 0x7d64000160fc by thread T2 (mutexes: write M1821):
    #0 ff_thread_report_progress src/libavcodec/pthread_frame.c:565 (ffmpeg+0x000000dcb08a)