gatt client: Fix SIGABRT

JIRA Issue: http://suprem.sec.samsung.net/jira/browse/TSAM-14037

[Problem] Application terminates when GATT descriptor
          'WriteValue' API fails.
[Cause] In case of error in BlueZ DBUS API 'WriteValue', NULL
        is received in g_dbus_connection_call_finish() in gatt
        client. So, instead of returning after chekcing
        error value, code was trying to unref NULL value causing
        application to terminate abnormally.
[Measure] Return from GATT write descriptor callback, in case of error.
[Checking Method] Perform GATT desc 'WriteValue' using bt_unit_test.

Change-Id: I9fd230e091e5a16618c8d36ebbe94bafc768ef92
Signed-off-by: Anupam Roy <anupam.r@samsung.com>