assrender: fix some leaks

g_slist_remove_*() removes the list node without freeing it.
8 jobs for assrender-leaks in 55 minutes and 51 seconds (queued for 17 seconds)
Status Job ID Name Coverage
  Preparation
passed #162670
gst indent

00:00:45

passed #162669
manifest

00:00:55

 
  Build
passed #162671
build fedora x86_64

00:05:57

 
  Test
passed #162672
check fedora

00:05:32

 
  Full Builds
passed #162673
build android arm64 api28

00:05:48

passed #162675
build cerbero cross win32

00:12:26

passed #162676
build cerbero cross win64

00:12:00

passed #162674
build cerbero fedora x86_64

00:12:25