RDMA/cxgb4: Fix potential null-ptr-deref in pass_establish()

RDMA/cxgb4: Fix potential null-ptr-deref in pass_establish()

If get_ep_from_tid() fails to lookup non-NULL value for ep, ep is
dereferenced later regardless of whether it is empty.
This patch adds a simple sanity check to fix the issue.

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

Fixes: 944661dd97f4 ("RDMA/iw_cxgb4: atomically lookup ep and get a reference")
Signed-off-by: Nikita Zhandarovich <n.zhandarovich@fintech.ru>
Link: https://lore.kernel.org/r/20230202184850.29882-1-n.zhandarovich@fintech.ru
Signed-off-by: Leon Romanovsky <leon@kernel.org>