northd, ovn-sbctl: Fix null pointer dereferences in sbrec delete calls.

[oilbeater]

We experienced some segmentation faults in our end-to-end tests, and with these patches, the segmentation fault has been resolved.

Add missing NULL checks before calling sbrec delete functions to prevent null pointer dereferences:

• en-sync-sb.c: Add missing 'continue' after free(sb_lb) in sync_changed_lbs() to avoid use-after-free when accessing sb_lb->sbrec_lb in the subsequent sync_sb_lb_record() call.
• northd.c: Add NULL check for op->sb before calling sbrec_port_binding_delete() in ls_handle_lsp_changes().
• northd.c: Add NULL check for bfd_e->sb_bt before calling sbrec_bfd_delete() in bfd_table_sync().
• ovn-sbctl.c: Add NULL check for encaps[i] before calling sbrec_encap_delete() in cmd_chassis_del().

[almusil]

Hi,
thank you for the patch.

We use mailing list to submit patches https://docs.ovn.org/en/latest/internals/contributing/submitting-patches.html.

Each of the changes should be a separate commit with proper Fixes tag that would help us with tracking how far we need to make the backport. Ideally each change would have a test added that will exercise the scenario when the crash occurs to prevent regressions in the future.