shared/gatt-db: Fix invalid read
shared/gatt-db: Fix invalid read

notify_list has to be reset to NULL otherwise gatt_db_service_destroy may
still attempt to access it:

Invalid read of size 4
   at 0x4166E5: queue_isempty (queue.c:398)
   by 0x41489B: gatt_db_service_destroy (gatt-db.c:235)
   by 0x416650: queue_remove_all (queue.c:375)
   by 0x4166A4: queue_destroy (queue.c:81)
   by 0x414817: gatt_db_unref (gatt-db.c:273)
   by 0x40B5F3: destroy_context (test-gatt.c:587)
   by 0x40C132: test_server (test-gatt.c:602)
   by 0x4E9E5E0: ??? (in /usr/lib64/libglib-2.0.so.0.3800.2)
   by 0x4E9E7A5: ??? (in /usr/lib64/libglib-2.0.so.0.3800.2)
   by 0x4E9E7A5: ??? (in /usr/lib64/libglib-2.0.so.0.3800.2)
   by 0x4E9E7A5: ??? (in /usr/lib64/libglib-2.0.so.0.3800.2)
   by 0x4E9EB1A: g_test_run_suite (in /usr/lib64/libglib-2.0.so.0.3800.2)
 Address 0x577bfa8 is 24 bytes inside a block of size 32 free'd
   at 0x4C28577: free (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
   by 0x414807: gatt_db_unref (gatt-db.c:272)
   by 0x40B5F3: destroy_context (test-gatt.c:587)
   by 0x40C132: test_server (test-gatt.c:602)
   by 0x4E9E5E0: ??? (in /usr/lib64/libglib-2.0.so.0.3800.2)
   by 0x4E9E7A5: ??? (in /usr/lib64/libglib-2.0.so.0.3800.2)
   by 0x4E9E7A5: ??? (in /usr/lib64/libglib-2.0.so.0.3800.2)
   by 0x4E9E7A5: ??? (in /usr/lib64/libglib-2.0.so.0.3800.2)
   by 0x4E9EB1A: g_test_run_suite (in /usr/lib64/libglib-2.0.so.0.3800.2)
   by 0x40A9D7: main (test-gatt.c:1216)

pstglia/external-bluetooth-bluez,mapfau/bluez,ComputeCycles/bluez,pkarasev3/bluez,ComputeCycles/bluez,mapfau/bluez,pkarasev3/bluez,pstglia/external-bluetooth-bluez,pkarasev3/bluez,pkarasev3/bluez,mapfau/bluez,silent-snowman/bluez,ComputeCycles/bluez,ComputeCycles/bluez,pstglia/external-bluetooth-bluez,silent-snowman/bluez,mapfau/bluez,pstglia/external-bluetooth-bluez,silent-snowman/bluez,silent-snowman/bluez