Fix `err != 0` uses

core/image/png/example.odin

@@ -213,7 +213,7 @@ write_image_as_ppm :: proc(filename: string, image: ^image.Image) -> (success: b
 	}
 
 	fd, err := open(filename, flags, mode)
-	if err != 0 {
+	if err != nil {
 		return false
 	}
 	defer close(fd)

core/image/png/helpers.odin

@@ -450,7 +450,7 @@ when false {
 		}
 
 		fd, fderr := open(filename, flags, mode)
-		if fderr != 0 {
+		if fderr != nil {
 			return .Cannot_Open_File
 		}
 		defer close(fd)