Fix race condition in InternalFFMpegRegister initialization.

[vrabaud]

initLogger_ does not check if the logger has been initizalized before and it might initialize it several times from different threads, racing with other threads that are logging.

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[vrabaud]

FYI, the stack trace we have is (line numbers might be off by a few units; if needed I can get the exact code references):

  Write of size 4 at 0x556a7df5e66c by thread T152 (mutexes: write M0):
    #0 av_log_set_level third_party/ffmpeg/src/libavutil/log.c:387:18 
    #1 initLogger_ third_party/OpenCV/public/modules/./videoio/src/cap_ffmpeg_impl.hpp:950:13
    #2 InternalFFMpegRegister::init(bool) third_party/OpenCV/public/modules/./videoio/src/cap_ffmpeg_impl.hpp:930:9
    #3 CvVideoWriter_FFMPEG::open(char const*, int, double, int, int, cv::VideoWriterParameters const&) third_party/OpenCV/public/modules/./videoio/src/cap_ffmpeg_impl.hpp:2779:5
    #4 cvCreateVideoWriterWithParams_FFMPEG third_party/OpenCV/public/modules/./videoio/src/cap_ffmpeg_impl.hpp:3344:17
...
  Previous read of size 4 at 0x556a7df5e66c by thread T190 (mutexes: write M1):
    #0 av_log_default_callback third_party/ffmpeg/src/libavutil/log.c:316:17
    #1 av_vlog third_party/ffmpeg/src/libavutil/log.c:377:9
    #2 av_log third_party/ffmpeg/src/libavutil/log.c:369:5
    #3 h264_parse_nal_header third_party/ffmpeg/src/libavcodec/h2645_parse.c:324:5