shared/hci: Fix crash when bt_hci_unref is used from callback

shared/hci: Fix crash when bt_hci_unref is used from callback

If application unref its reference on the command callback it can lead
to crashes like the following:

 Invalid read of size 1
    at 0x254270: wakeup_writer (hci.c:187)
    by 0x254321: process_response (hci.c:229)
    by 0x254590: process_event (hci.c:263)
    by 0x254590: io_read_callback (hci.c:305)
    by 0x269258: watch_callback (io-glib.c:170)
    by 0x496756F: g_main_context_dispatch (in /usr/lib64/libglib-2.0.so.0.6200.6)
    by 0x49678FF: ??? (in /usr/lib64/libglib-2.0.so.0.6200.6)
    by 0x4967BF2: g_main_loop_run (in /usr/lib64/libglib-2.0.so.0.6200.6)
    by 0x269C6C: mainloop_run (mainloop-glib.c:79)
    by 0x26A219: mainloop_run_with_signal (mainloop-notify.c:201)
    by 0x171A35: main (main.c:770)
  Address 0x53b7e81 is 17 bytes inside a block of size 64 free'd
    at 0x483AA0C: free (vg_replace_malloc.c:540)
    by 0x254A70: bt_hci_unref (hci.c:461)

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