Revert "Cleanup: Delete dead code from send_merge_thread()"

This reverts commit fb823de due to a regression.  It is in fact possible
for the range->eos_marker to be false on error.

Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue openzfs#14042
Closes openzfs#14104

module/zfs/dmu_send.c

@@ -1586,8 +1586,9 @@ send_merge_thread(void *arg)
 		}
 		range_free(front_ranges[i]);
 	}
-	ASSERT3P(range, !=, NULL);
-	ASSERT3S(range->eos_marker, ==, B_TRUE);
+	if (range == NULL)
+		range = kmem_zalloc(sizeof (*range), KM_SLEEP);
+	range->eos_marker = B_TRUE;
 	bqueue_enqueue_flush(&smt_arg->q, range, 1);
 	spl_fstrans_unmark(cookie);
 	thread_exit();