db/view: view_builder::execute(): only inject partition-start if needed

When resuming a build-step, the view builder injects the partition-start
fragment of the last processed partition, to bring the consumer
(compactor) into the correct state before it starts to consume the
remainder of the partition content. This results in an invalid fragment
stream when the partition was actually over or there is nothing left for
the build step. Make the inject conditional on when the reader contains
more data for the partition.

Fixes: #11607

db/view/view.cc

@@ -2261,7 +2261,9 @@ void view_builder::execute(build_step& step, exponential_backoff_retry r) {
             batch_size,
             query::max_partitions,
             view_builder::consumer{*this, step, now});
-    consumer.consume_new_partition(step.current_key); // Initialize the state in case we're resuming a partition
+    if (auto mfp = step.reader.peek().get(); mfp && !mfp->is_partition_start()) {
+        consumer.consume_new_partition(step.current_key); // Initialize the state in case we're resuming a partition
+    }
     auto built = step.reader.consume_in_thread(std::move(consumer));
 
     _as.check();