processing: Fix use-after-free

The buffer holding the last output line got freed when it was converted into a pointer and then returned.

This fixes that while also dropping the check for valid UTF-8 in the string returned by the post-processor. Let the next part of the pipeline deal with it.
4 jobs for devel in 4 minutes and 8 seconds (queued for 1 second)
Status Name Job ID Coverage
  Package
passed package-pureos-byzantium #376365
librem5

00:01:29

passed package-pureos-byzantium:arm64 #376366
librem5:arm64

00:04:07

 
  Test Package
passed autopkgtest-pureos-byzantium-package #376367
librem5

00:00:50

passed lintian-pureos-byzantium-package #376368
librem5

00:00:49