RDMA/nes: Don't call event handler if pointer is NULL

Don't call the ibqp event_handler pointer in the case it wasn't initialized.

Signed-off-by: Tatyana Nikolova <Tatyana.E.Nikolova@intel.com>
Signed-off-by: Donald Wood <Donald.E.Wood@intel.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>