ssa.c: In ssa_ctrl_stop, avoid blocking in case of uninitialized socket pair

If ibssa application is exiting as result after
ib device opening failure, the sock is uninitialized.
IO operation on unitized socket could crash or block the
application.

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