RDMA/cxgb4: Added NULL check for lookup_atid

RDMA/cxgb4: Added NULL check for lookup_atid

The lookup_atid() function can return NULL if the ATID is
invalid or does not exist in the identifier table, which
could lead to dereferencing a null pointer without a
check in the `act_establish()` and `act_open_rpl()` functions.
Add a NULL check to prevent null pointer dereferencing.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: cfdda9d76436 ("RDMA/cxgb4: Add driver for Chelsio T4 RNIC")
Signed-off-by: Mikhail Lobanov <m.lobanov@rosalinux.ru>
Link: https://patch.msgid.link/20240912145844.77516-1-m.lobanov@rosalinux.ru
Signed-off-by: Leon Romanovsky <leon@kernel.org>