net/bluetooth: avoid NULL pointer dereference when there is an error during setup

net/bluetooth: avoid NULL pointer dereference when there is an error during setup

If there is an error while connecting a SCO socket, the state machine
transitions directly to BT_DISCONN state in __sco_sock_close before some
pointers are set non-NULL.  The socket shutdown code assumes that these
pointers are non-NULL in BT_DISCONN state, and dereferences NULL.

My Jabra CLIPPER headset triggers this bug on nearly every connection.

This patch checks that the pointers are non-NULL before attempting to
dereference them.  It has kept my kernels NULL-dereference-free for
almost a month.

Example backtrace:

May 14 09:30:32 faye bluetoothd[14989]: audio/transport.c:media_request_reply() Request Acquire Reply Input/output error
May 14 09:30:32 faye bluetoothd[14989]: audio/transport.c:media_owner_free() Owner :1.19
May 14 09:30:32 faye bluetoothd[14989]: audio/transport.c:media_owner_remove() Owner :1.19 Request Acquire
May 14 09:30:32 faye kernel: [127767.124812] sco_sock_shutdown:769: sock d75e3680, sk f399da00
May 14 09:30:32 faye kernel: [127767.124829] sco_sock_clear_timer:94: sock f399da00 state 1
May 14 09:30:32 faye kernel: [127767.124841] __sco_sock_close:364: sk f399da00 state 1 socket d75e3680
May 14 09:30:32 faye kernel: [127767.124853] sco_sock_set_timer:88: sock f399da00 state 8 timeout 2000
May 14 09:30:32 faye kernel: [127767.124920] BUG: unable to handle kernel NULL pointer dereference at 00000008
(cherry picked from commit 587acb9d3a3a89bf3d3b0ba31926cd6fff8a1d5f)