ssa.c: In ssa_stop_access, handle NULL SMDB reference object

If ibssa application couldn't open ib devices and exits,
smdb pointer is NULL. This can cause crash.

Signed-off-by: Sasha Kotchubievsky <sashakot@mellanox.com>
Signed-off-by: Hal Rosenstock <hal@mellanox.com>
ssa.c: In ssa_close_devices, call ibv_close_device only if verbs pointer is not NULL

If ibssa application couldn't open ib devices and exits,
verbs pointer is NULL.

Signed-off-by: Sasha Kotchubievsky <sashakot@mellanox.com>
Signed-off-by: Hal Rosenstock <hal@mellanox.com>