timezone: Fix an uninitialized use

GTK ci rightly complains about this when ti builds
GLib as a subproject with -Werror=maybe-uninitialized.

subkey_dynamic_w will be freed without being initialized
when the first goto is taken.
13 jobs for fix-uninitialized-use in 11 minutes and 12 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Style Check
passed #835571
check-todos

00:02:03

passed #835570
style-check-diff

00:00:59

 
  Build
passed #835574
cross-android_api21_arm64

00:01:11

passed #835575
cross-android_api28_arm64

00:01:36

passed #835576
cross-mingw64

00:03:27

passed #835573
debian-stable-x86_64

00:03:42

passed #835572
fedora-x86_64

00:08:08

passed #835579
freebsd-11
freebsd-11-x86_64

00:05:25

passed #835580
freebsd-12
freebsd-12-x86_64

00:05:07

passed #835581
macos
macos

00:04:29

passed #835577
win32-ps
msys2-mingw32

00:06:34

passed #835578
win32-ps
vs2017-x64

00:08:05

 
  Coverage
passed #835582
coverage

00:03:02

78.4%