Bluetooth: Check if the hci connection exists in SCO shutdown

Bluetooth: Check if the hci connection exists in SCO shutdown

Checking only for sco_conn seems to not be enough and lead to NULL
dereferences in the code, check for hcon instead.

<1>[11340.226404] BUG: unable to handle kernel NULL pointer dereference at
0000000
8
<4>[11340.226619] EIP is at __sco_sock_close+0xe8/0x1a0
<4>[11340.226629] EAX: f063a740 EBX: 00000000 ECX: f58f4544 EDX: 00000000
<4>[11340.226640] ESI: dec83e00 EDI: 5f9a081f EBP: e0fdff38 ESP: e0fdff1c
<0>[11340.226674] Stack:
<4>[11340.226682]  c184db87 c1251028 dec83e00 e0fdff38 c1754aef dec83e00
00000000
e0fdff5c
<4>[11340.226718]  c184f587 e0fdff64 e0fdff68 5f9a081f e0fdff5c c1751852
d7813800
62262f10
<4>[11340.226752]  e0fdff70 c1753c00 00000000 00000001 0000000d e0fdffac
c175425c
00000041
<0>[11340.226793] Call Trace:
<4>[11340.226813]  [<c184db87>] ? sco_sock_clear_timer+0x27/0x60
<4>[11340.226831]  [<c1251028>] ? local_bh_enable+0x68/0xd0
<4>[11340.226846]  [<c1754aef>] ? lock_sock_nested+0x4f/0x60
<4>[11340.226862]  [<c184f587>] sco_sock_shutdown+0x67/0xb0
<4>[11340.226879]  [<c1751852>] ? sockfd_lookup_light+0x22/0x80
<4>[11340.226897]  [<c1753c00>] sys_shutdown+0x30/0x60
<4>[11340.226912]  [<c175425c>] sys_socketcall+0x1dc/0x2a0
<4>[11340.226929]  [<c149ba78>] ? trace_hardirqs_on_thunk+0xc/0x10
<4>[11340.226944]  [<c18860f1>] syscall_call+0x7/0xb
<4>[11340.226960]  [<c1880000>] ? restore_cur+0x5e/0xd7
<0>[11340.226969] Code: <f0> ff 4b 08 0f 94 c0 84 c0 74 20 80 7b 19 01 74
2f b8 0a 00 00

Reported-by: Chuansheng Liu <chuansheng.liu@intel.com>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>