curlhttpsink: free content type before assigning it

Not doing this would result in a memory leak.
6 jobs for merge_free_content_type in 31 minutes and 30 seconds (queued for 20 minutes and 23 seconds)
latest
Status Job ID Name Coverage
  Preparation
passed #65668
gst indent

00:00:28

passed #65667
manifest

00:00:19

 
  Build
passed #65669
build fedora x86_64

00:03:36

 
  Test
passed #65670
check fedora

00:16:11

 
  Full Builds
passed #65671
build android arm64 api28

00:02:27

passed #65672
build cerbero fedora x86_64

00:10:32