Prevents wrap-around to negative values while calculating the duration string.

Before:

    Duration: -411422:-59:-42.17, start: 0.000000, bitrate: 0 kb/s

After:

    Duration: 781623:28:34.17, start: 0.000000, bitrate: 0 kb/s
Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>