Do not clear pending if points to the checkpoint

[deadtrickster]

With Rav3 Tanzu DQ's log writer errors as soon as snapshot is requested. For example,

2026-03-25 22:50:44.857425+01:00 [warning] <0.1364.0> segment_writer: missing index 313868 in mem table for uid DQ_2F_57D0UL0CNGN2start index 313868 checking to see if UId has been unregistered
2026-03-25 22:50:44.857523+01:00 [error] <0.1364.0> segment_writer: uid <redacted> is registered, exiting...
2026-03-25 22:50:44.857629+01:00 [error] <0.1341.0> segment_writer: 1 failures encountered during segment flush. Errors: [{{<<"redacted">>,
2026-03-25 22:50:44.857629+01:00 [error] <0.1341.0>                                                                         [{#Ref<0.588533474.2887385091.231957>,
2026-03-25 22:50:44.857629+01:00 [error] <0.1341.0>                                                                           [{313868,
2026-03-25 22:50:44.857629+01:00 [error] <0.1341.0>                                                                             699049}]}]},
2026-03-25 22:50:44.857629+01:00 [error] <0.1341.0>                                                                        {missing_index,
2026-03-25 22:50:44.857629+01:00 [error] <0.1341.0>                                                                         <<"redacted">>,
2026-03-25 22:50:44.857629+01:00 [error] <0.1341.0>                                                                         313868}}]

initially I thought it is due to the [] being the default for live indexes, but looks like it is due to some race here, in complete_snapshot.

[michaelklishin]

rabbitmq/rabbitmq-server#15837