arsd png bug

[Joerg Joergonson via Digitalmars-d-learn]

1810:
		case 3:
			auto arr = data.dup;
			foreach(i; 0 .. arr.length) {
				auto prev = i < bpp ? 0 : arr[i - bpp];
				if (i >= previousLine.length) break;
				arr[i] += cast(ubyte)
					/*std.math.floor*/( cast(int) (prev + previousLine[i]) / 2);
			}


adding
	if (i >= previousLine.length) break;

prevents some crashes and seems to work.