kdbus: connection: fix handling of failed fget()

kdbus: connection: fix handling of failed fget()

The patch 5fc8dd5c84fc: "kdbus: add connection, queue handling and
message validation code" from Sep 11, 2014, leads to the following
static checker warning:

	ipc/kdbus/connection.c:2000 kdbus_cmd_send()
	warn: 'cancel_fd' isn't an ERR_PTR

Fix this by checking for NULL pointers returned from fget().

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Daniel Mack <daniel@zonque.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Paul Osmialowski <p.osmialowsk@samsung.com>