WinGui:  Don't crash if a corrupted source file causes the duration to be mis-detected. Note, in this case duration of the title / chapters will show as 00:00:00 and no chapters may display.

git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@5539 b64f7644-9d1e-0410-96f1-a4d463321fa5