IB/core: Verify that QP handler is valid before dispatching events

For QPs of type IB_QPT_XRC_TGT the IB core assigns a common event
handler __ib_shared_qp_event_handler(), and the optionally supplied
event handler is stored. When the common handler is called it iterates
on all opened QPs and calles the original handlers without checking if
they are NULL.  Fix that.

Signed-off-by: Shlomo Pongratz <shlomop@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>