packer: Fix possible array indexing error when pixels is null

[hexaredecimal]

Saw this bug while watching the stream on YT. The bug is that pixels is indexed before the null check and that could result in a fatal error in case of a null value. The null check is done a little bit late.

koil/packer.c3

Lines 24 to 32 in 48d0f0d

	char *pixels = stbi_load((ZString)filename, &x, &y, null, comp);
	int size = x*y*comp;
	usz offset = pack.len();
	assets.push({filename, offset, x, y});
	pack.append_chars((String)pixels[0..size-1]);
	if (pixels == null) {
	io::printf("ERROR: could not load file %s\n", filename);
	return 1;
	}

What I did

Moved the lines for getting the offset, size, appending to assets and appending pixels to the pack to run after the null check.