DAOS-18264 pool: non-NULL check for sp_map (#17216)

[NiuYawei]

The ds_pool::sp_map could be NULL if the pool map hasn't been populated, a non-NULL check must be performed before accessing.

Steps for the author:

• [ ] Commit message follows the guidelines.
• [ ] Appropriate Features or Test-tag pragmas were used.
• [ ] Appropriate Functional Test Stages were run.
• [ ] At least two positive code reviews including at least one code owner from each category referenced in the PR.
• [ ] Testing is complete. If necessary, forced-landing label added and a reason added in a comment.

After all prior steps are complete:

• [ ] Gatekeeper requested (daos-gatekeeper added as a reviewer).

[github-actions[bot]]

Ticket title is 'Aurora daos_perf: Server got crashed when try to set NVMe to faulty' Status is 'Awaiting backport' Labels: 'ALCF,alcf_prod,request_for_2.6.5' https://daosio.atlassian.net/browse/DAOS-18264