drbd: abort drbd_start_resync if there is no connection

This was found by a static analysis tool. While highly unlikely, be sure
to return without dereferencing the NULL pointer.

Reported-by: Shaobo <shaobo@cs.utah.edu>
Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>