ass_blur: check for memory allocation size overflows
ass_blur: check for memory allocation size overflows

Check for overflows that could happen with alignment and the
multiplication. The INT_MAX / 4 is somewhat approximate and assumes that
degenerate alignment values won't happen.

This still assumes that a possibly overflowing end_w/end_h calculation
doesn't make the compiler's optimizer destroy the overflow checks.

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