Bluetooth: L2CAP: handle NULL sock pointer in l2cap_sock_alloc

commit 5f397409f8ee5bc82901eeaf799e1cbc4f8edcf1 upstream.

A NULL sock pointer is passed into l2cap_sock_alloc() when it is called
from l2cap_sock_new_connection_cb() and the error handling paths should
also be aware of it.

Seemingly a more elegant solution would be to swap bt_sock_alloc() and
l2cap_chan_create() calls since they are not interdependent to that moment
but then l2cap_chan_create() adds the soon to be deallocated and still
dummy-initialized channel to the global list accessible by many L2CAP
paths. The channel would be removed from the list in short period of time
but be a bit more straight-forward here and just check for NULL instead of
changing the order of function calls.

Found by Linux Verification Center (linuxtesting.org) with SVACE static
analysis tool.

Fixes: 7c4f78cdb8e7 ("Bluetooth: L2CAP: do not leave dangling sk pointer on error in l2cap_sock_create()")
Cc: stable@vger.kernel.org
Signed-off-by: Fedor Pchelkin <pchelkin@ispras.ru>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>