shared/att: Fix another memory leak
shared/att: Fix another memory leak

In case of bt_att_cancel_all before resetting destroy callback to NULL
it needs to be called so the requestor can properly free the resources:

104 (40 direct, 64 indirect) bytes in 1 blocks are definitely lost in loss record 47 of 55
   at 0x4C291D4: calloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
   by 0x408359: bt_gatt_exchange_mtu (gatt-helpers.c:479)
   by 0x405345: bt_gatt_client_new (gatt-client.c:1196)
   by 0x401A4B: test_client (test-gatt.c:198)
   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 0x40162E: main (test-gatt.c:265)

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