ass_shaper_find_runs: don't decrement pointer beyond array start
ass_shaper_find_runs: don't decrement pointer beyond array start

That triggers undefined behavior.

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