drbd: Fix future possible NULL pointer dereference

Right now every resource has exactly one connection. But we are preparing
for dynamic connections. I.e. in the future thre can be resources without
connections.

However smatch points this out as 'variable dereferenced before check',
which is correct.

This issue was introduced in
drbd: get_one_status(): Iterate over resource->devices instead of connection->peer_devices

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Andreas Gruenbacher <agruen@linbit.com>
Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Jens Axboe <axboe@fb.com>