screenshot: fix invalid free created by the previous change

The argv length is static and there is a custom loop to free parameter
which needs to have its index updated
4 jobs for master in 8 minutes and 24 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
passed #132824
build-fedora

00:05:39

passed #132825
flatpak

00:07:58

 
  Review
passed #132826
review

00:00:23

manual #132827
allowed to fail manual
stop_review