ssa_path_record.c: Init PRDB pointer to NULL in ssa_pr_compute_half_world

If PR calculation fails and caller dosen't check the return value,
it may access destroyed PRDB instance. NULL pointer will cause
a segmentation fault that is easier for debugging.

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