ass.c: add assert to silence analyzer warning
ass.c: add assert to silence analyzer warning

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