Update Windows builds to FFmpeg 4.1

libs/windows/include/libavutil/file.h

@@ -33,6 +33,8 @@
  * allocated buffer or map it with mmap() when available.
  * In case of success set *bufptr to the read or mmapped buffer, and
  * *size to the size in bytes of the buffer in *bufptr.
+ * Unlike mmap this function succeeds with zero sized files, in this
+ * case *bufptr will be set to NULL and *size will be set to 0.
  * The returned buffer must be released with av_file_unmap().
  *
  * @param log_offset loglevel offset used for logging