Fix freezed proxy frames

`ffmpeg_generic_seek_workaround()` applied negative offset for seqrched
packet timestamp, but proxies always start from 0 and timestamp can be
 negative.

Limit timestamp value to 0, because `av_seek_frame()` doesn't accept
negative timestamps and returns with error. This prevents seeking from
working correctly.

source/blender/imbuf/intern/anim_movie.c

@@ -1140,6 +1140,7 @@ static int ffmpeg_generic_seek_workaround(struct anim *anim, int64_t requested_p
    * small. */
   for (int offset = 5; offset < 25; offset++) {
     current_pos = requested_pos - ((int64_t)(offset)*AV_TIME_BASE / frame_rate);
+    current_pos = max_ii(current_pos, 0);
 
     /* Seek to timestamp. */
     if (av_seek_frame(anim->pFormatCtx, -1, current_pos, AVSEEK_FLAG_BACKWARD) < 0) {