Bluetooth: L2CAP: handle NULL sock pointer in l2cap_sock_alloc

In the Linux kernel, the following vulnerability has been resolved: Bluetooth: L2CAP: handle NULL sock pointer in l2cap_sock_alloc 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.