Fix crash issue

Fix crash issue

In the case of using the preference api after the global resource is destroyed,
a problem occurs by accessing the destroyed pref object.
This patch fixes that issue.

01-01 12:34:23.143+0900 I/CAPI_APPFW_PREFERENCE(P 5722, T 5722): file-backend-internal.cc: ~FileBackend(33) >
01-01 12:34:23.143+0900 I/CAPI_APPFW_PREFERENCE(P 5722, T 5722): file-backend-internal.cc: RemoveWatch(173) >
01-01 12:34:23.206+0900 I/CAPI_APPFW_PREFERENCE(P 5722, T 5742): stub.cc: preference_unset_changed_cb(317) >

Change-Id: I685c3d39ac0ee6e674d91f44b14c8d965edd08bf
Signed-off-by: Changgyu Choi <changyu.choi@samsung.com>