shared/gatt: Fix NULL pointer dereference

shared/gatt: Fix NULL pointer dereference

bluetoothd[363094]: src/device.c:device_connect_le() Connection attempt to: 00:AA:01:00:00:23

Program received signal SIGSEGV, Segmentation fault.
write_complete_cb (attr=0x55555580aa30, err=-110, user_data=0x55555585f7c0) at src/shared/gatt-server.c:793
793             util_debug(server->debug_callback, server->debug_data,
(gdb) bt
 #0  write_complete_cb (attr=0x55555580aa30, err=-110, user_data=0x55555585f7c0) at src/shared/gatt-server.c:793
 #1  0x00005555556a5852 in pending_write_result (p=0x555555866030, err=<optimized out>) at src/shared/gatt-db.c:162
 #2  0x00005555556a5ac7 in write_timeout (user_data=0x555555866030) at src/shared/gatt-db.c:1879
 #3  0x00005555556a9b15 in timeout_callback (user_data=user_data@entry=0x555555864b20) at src/shared/timeout-glib.c:34
 #4  0x00007ffff7e1f081 in g_timeout_dispatch (source=source@entry=0x555555864f00, callback=0x5555556a9b00 <timeout_callback>, user_data=0x555555864b20) at ../glib/gmain.c:4705
 #5  0x00007ffff7e1e570 in g_main_dispatch (context=0x5555557d9630) at ../glib/gmain.c:3216
 #6  g_main_context_dispatch (context=context@entry=0x5555557d9630) at ../glib/gmain.c:3881
 #7  0x00007ffff7e1e900 in g_main_context_iterate (context=0x5555557d9630, block=block@entry=1, dispatch=dispatch@entry=1, self=<optimized out>) at ../glib/gmain.c:3954
 #8  0x00007ffff7e1ebf3 in g_main_loop_run (loop=0x5555557d75d0) at ../glib/gmain.c:4148
 #9  0x00005555556a9dbd in mainloop_run () at src/shared/mainloop-glib.c:79
 #10 0x00005555556aa36a in mainloop_run_with_signal (func=<optimized out>, user_data=0x0) at src/shared/mainloop-notify.c:201
 #11 0x00005555555bb9e3 in main (argc=<optimized out>, argv=<optimized out>) at src/main.c:770

Signed-off-by: Anuj Jain <anuj01.jain@samsung.com>
Signed-off-by: Ayush Garg <ayush.garg@samsung.com>