Fix integer overflow while parsing \fad(arg, large negative number)
Fix integer overflow while parsing \fad(arg, large negative number)

If t3 is initially negative, it should be set to a value larger
than the duration of the event. This triggers the `now < t3` branch
in interpolate_alpha (if none of the earlier branches are taken).
The same effect can be achieved by setting t3 to the duration itself.

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=531.
Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=3905.
Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=11736.

xbmc/libass,MrSmile/libass,xbmc/libass,ShiftMediaProject/libass,ShiftMediaProject/libass,libass/libass,MrSmile/libass,astiob/libass,Akemi/libass,libass/libass,Akemi/libass,astiob/libass,Akemi/libass