gbookmarkfile: Fix some minor leaks when metadata elements are repeated

oss-fuzz#9674

Signed-off-by: Philip Withnall <withnall@endlessm.com>
8 jobs for bookmark-file-leak in 19 minutes and 5 seconds (queued for 12 seconds)
Status Job ID Name Coverage
  Build
passed #76880
cross-android_api21_arm64

00:01:44

passed #76881
cross-android_api28_arm64

00:01:37

passed #76882
cross-mingw64

00:02:07

passed #76879
fedora-x86_64

00:04:23

manual #76885
freebsd-11 allowed to fail manual
freebsd-11-x86_64
passed #76883
win32
msys2-mingw32

00:07:35

passed #76884
win32
vs2017-x64

00:06:04

 
  Coverage
passed #76886
coverage

00:01:36

73.3%