carousel: Fix crash when attempting to scroll on an empty carousel

If there are no pages in a carousel, abandon the attempt at scrolling through
it.

Fixes #231
4 jobs for 231 in 3 minutes and 14 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Build
passed abi-check #1399113

00:01:47

passed api-visibility #1399110

00:00:28

passed build-flatpak #1399112
flatpak

00:03:14

passed doc #1399111
flatpak

00:01:06