gatt-server: Fix crash while disconnecting

gatt-server: Fix crash while disconnecting

If there is a pending notify multiple the code was not removing before
freeing the object causing the following crash:

Invalid read of size 8
   at 0x4A3D10: notify_multiple (gatt-server.c:1703)
   by 0x4D05F0: timeout_callback (timeout-glib.c:25)
   by 0x4956900: ??? (in /usr/lib64/libglib-2.0.so.0.7000.5)
   by 0x49560AE: g_main_context_dispatch
   (in /usr/lib64/libglib-2.0.so.0.7000.5)
   by 0x49AB307: ??? (in /usr/lib64/libglib-2.0.so.0.7000.5)
   by 0x49557C2: g_main_loop_run
   (in /usr/lib64/libglib-2.0.so.0.7000.5)
   by 0x4D0A34: mainloop_run (mainloop-glib.c:66)
   by 0x4D0F2B: mainloop_run_with_signal (mainloop-notify.c:188)
   by 0x2B0CD1: main (main.c:1276)
 Address 0x6ca35c8 is 136 bytes inside a block of size 144 free'd
   at 0x48470E4: free (vg_replace_malloc.c:872)
   by 0x415E73: gatt_server_cleanup (device.c:698)
   by 0x415E73: attio_cleanup (device.c:715)
   by 0x47745B: queue_foreach (queue.c:207)
   by 0x490C54: disconnect_cb (att.c:701)
   by 0x4CF4AF: watch_callback (io-glib.c:157)
   by 0x49560AE: g_main_context_dispatch
   (in /usr/lib64/libglib-2.0.so.0.7000.5)
   by 0x49AB307: ??? (in /usr/lib64/libglib-2.0.so.0.7000.5)
   by 0x49557C2: g_main_loop_run
   (in /usr/lib64/libglib-2.0.so.0.7000.5)
   by 0x4D0A34: mainloop_run (mainloop-glib.c:66)
   by 0x4D0F2B: mainloop_run_with_signal (mainloop-notify.c:188)
   by 0x2B0CD1: main (main.c:1276)

Signed-off-by: Manika Shrivastava <manika.sh@samsung.com>
Signed-off-by: Ayush Garg <ayush.garg@samsung.com>