vorbisdec: fix leak of header buffers

handle_header_buffer() does no take ownership of
the buffer passed.

Fixes leaks in various unit tests.
8 jobs for vorbisdec-headers-leak in 32 minutes and 56 seconds (queued for 3 minutes and 25 seconds)
Status Job ID Name Coverage
  Preparation
passed #155533
gst indent

00:00:40

passed #155532
manifest

00:00:44

 
  Build
passed #155534
build fedora x86_64

00:06:48

 
  Test
passed #155535
check fedora

00:04:25

 
  Full Builds
passed #155536
build android arm64 api28

00:05:07

passed #155538
build cerbero cross win32

00:07:41

passed #155539
build cerbero cross win64

00:06:11

passed #155537
build cerbero fedora x86_64

00:07:28