DAOS-19332 server: check non-NULL 'dtc'

[NiuYawei]

Check non-NULL 'dtc' in dss_get_module_info() to avoid assertion in following dss_module_key_get() call.

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]

Ticket title is 'Check non-NULL 'dtc' in dss_get_module_info()'
Status is 'Open'
Errors are Unknown component
https://daosio.atlassian.net/browse/DAOS-19332

[wangshilong]

Just wondering you hit the segfault in server running or by adding unit test.

[NiuYawei]

Just wondering you hit the segfault in server running or by adding unit test.

See DAOS-19319: When the sysdb is incompatible format, vos_pool_open_metrics() will try to raise a RAS event, where dss_get_module_info() is called before TLS key is set.

VOS is per-thread and relies heavily on TLS, but sysdb is a global DB without it. We've already fixed several TLS issues in sysdb, and this looks like another one. :)