GTimeZone: fix non-threadsafe refcounting
GTimeZone: fix non-threadsafe refcounting

In the previous code, if the timezone was pulled out of the cache again
just as the last reference was being dropped, the cache code will
increase its refcount and return it while the unref code was freeing it.

Protect against that.

Closes #646435.

bluez-android/glib,cention-sany/glib,justinkb/aosp-bluez.glib,bluez-android/glib,MathieuDuponchelle/glib,bluez-android/glib,iConsole/Console-OS_external_bluetooth_glib,cosimoc/glib,justinkb/aosp-bluez.glib,lukasz-skalski/glib,johne53/MB3Glib,ieei/glib,ieei/glib,MathieuDuponchelle/glib,pstglia/external-bluetooth-glib,01org/android-bluez-glib,bluez-android/glib,cosimoc/glib,darren-clark/android_platform_external_bluetooth_glib,lukasz-skalski/glib,darren-clark/android_platform_external_bluetooth_glib,endlessm/glib,ieei/glib,pstglia/external-bluetooth-glib,Distrotech/glib,Distrotech/glib,zsx/glib,tchakabam/glib,endlessm/glib,cention-sany/glib,endlessm/glib,pstglia/platform-external-bluetooth-glib,djdeath/glib,MathieuDuponchelle/glib,iConsole/Console-OS_external_bluetooth_glib,gale320/glib,iConsole/Console-OS_external_bluetooth_glib,ieei/glib,krichter722/glib,johne53/MB3Glib,tamaskenez/glib,cention-sany/glib,djdeath/glib,gale320/glib,mzabaluev/glib,cosimoc/glib,Distrotech/glib,darren-clark/android_platform_external_bluetooth_glib,pstglia/platform-external-bluetooth-glib,antono/glib,justinkb/aosp-bluez.glib,darren-clark/android_platform_external_bluetooth_glib,johne53/MB3Glib,darren-clark/android_platform_external_bluetooth_glib,krichter722/glib,tamaskenez/glib,tchakabam/glib,antono/glib,tchakabam/glib,MathieuDuponchelle/glib,mzabaluev/glib,endlessm/glib,cosimoc/glib,tamaskenez/glib,pstglia/external-bluetooth-glib,tchakabam/glib,Distrotech/glib,endlessm/glib,gale320/glib,antono/glib,mzabaluev/glib,johne53/MB3Glib,Distrotech/glib,zsx/glib,gale320/glib,zsx/glib,lukasz-skalski/glib,tchakabam/glib,pstglia/platform-external-bluetooth-glib,djdeath/glib,pstglia/platform-external-bluetooth-glib,ieei/glib,cosimoc/glib,krichter722/glib,antono/glib,mzabaluev/glib,cention-sany/glib,01org/android-bluez-glib,justinkb/aosp-bluez.glib,pstglia/platform-external-bluetooth-glib,bluez-android/glib,tamaskenez/glib,krichter722/glib,djdeath/glib,mzabaluev/glib,lukasz-skalski/glib,pstglia/external-bluetooth-glib,justinkb/aosp-bluez.glib,cention-sany/glib,johne53/MB3Glib,01org/android-bluez-glib,01org/android-bluez-glib,krichter722/glib,zsx/glib,iConsole/Console-OS_external_bluetooth_glib,tamaskenez/glib,pstglia/external-bluetooth-glib,iConsole/Console-OS_external_bluetooth_glib,gale320/glib,01org/android-bluez-glib,lukasz-skalski/glib,MathieuDuponchelle/glib,djdeath/glib,johne53/MB3Glib