client: Fix crash when exiting

client: Fix crash when exiting

When exiting the available proxies are destroy in the same order they are
added causing the following crash when there are attributes whose service
has already been removed:

Invalid read of size 8
   at 0x414AAD: g_dbus_proxy_get_path (client.c:525)
   by 0x40B948: characteristic_is_child (gatt.c:136)
   by 0x40C420: gatt_remove_characteristic (gatt.c:157)
   by 0x4067A7: proxy_removed (main.c:446)
   by 0x414A2E: proxy_free (client.c:439)
   by 0x4E7AF6C: g_list_foreach (in /usr/lib64/libglib-2.0.so.0.4400.1)
   by 0x4E7AF8A: g_list_free_full (in /usr/lib64/libglib-2.0.so.0.4400.1)
   by 0x415D54: g_dbus_client_unref (client.c:1310)
   by 0x40511B: main (main.c:2067)
 Address 0x5eb5450 is 16 bytes inside a block of size 80 free'd
   at 0x4C29D6A: free (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
   by 0x4E8479E: g_free (in /usr/lib64/libglib-2.0.so.0.4400.1)
   by 0x4149D6: g_dbus_proxy_unref (client.c:517)
   by 0x414A8D: proxy_free (client.c:451)
   by 0x4E7AF6C: g_list_foreach (in /usr/lib64/libglib-2.0.so.0.4400.1)
   by 0x4E7AF8A: g_list_free_full (in /usr/lib64/libglib-2.0.so.0.4400.1)
   by 0x415D54: g_dbus_client_unref (client.c:1310)
   by 0x40511B: main (main.c:2067)