Do not encode images with illegal colormap indexes

examples.lua

@@ -164,3 +164,18 @@ local pixels = {
 	{ _, _, _, K, _, _, _ },
 }
 tga_encoder.image(pixels):save("colormapped_B8G8R8A8.tga", {colormap=colormap})
+
+-- encoding a colormapped image with illegal colormap indexes should error out
+local colormap = {
+	{   0,   0,   0,   0 },
+	{   0,   0,   0, 255 },
+}
+local status, message = pcall(
+	function ()
+		tga_encoder.image(pixels):encode({colormap=colormap})
+	end
+)
+assert(
+	false == status and
+	"init.lua:36: colormap index 2 not in colormap of size 2" == message
+)

init.lua

@@ -28,6 +28,18 @@ function image:encode_colormap_spec(properties)
 		colormap_pixel_depth = pixel_depth_by_color_format[
 			properties.color_format
 		]
+		-- ensure that each pixel references a legal colormap entry
+		for _, row in ipairs(self.pixels) do
+			for _, pixel in ipairs(row) do
+				local colormap_index = pixel[1]
+				if colormap_index >= #colormap then
+					error(
+						"colormap index " .. colormap_index ..
+						" not in colormap of size " .. #colormap
+					)
+				end
+			end
+		end
 	end
 	local colormap_spec =
 		string.char(0, 0) .. -- first entry index