RDMA/cxgb3: Don't post zero-byte read if endpoint is going away

tx_ack() wasn't checking the endpoint state and consequently would
attempt to post the p2p 0B read on an endpoint/QP that is closing or
aborting.  This causes a NULL pointer dereference crash.

Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>