ssa.c: Use pointers instead of pthread_t for access_thread and access_prdb_handler

Static, global pointers are initialized by 0. It allows for them
to be checked in destroy and to join only if the thread was actually
created.

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