Don't index tombstones.

patch by Mikhail Stepura; reviewed by Aleksey Yeschenko for CASSANDRA-7828

CHANGES.txt

@@ -1,4 +1,5 @@
 1.2.19
+ * Don't index tombstones (CASSANDRA-7828)
  * Don't allow compacted sstables to be marked as compacting (CASSANDRA-7145)
  * Track expired tombstones (CASSANDRA-7810)
  * Validate empty cell names from counter updates (CASSANDRA-7798)

src/java/org/apache/cassandra/db/index/SecondaryIndexManager.java

@@ -443,7 +443,7 @@ public class SecondaryIndexManager
             {
                 for (IColumn column : cf)
                 {
-                    if (index.indexes(column.name()))
+                    if (column.isLive() && index.indexes(column.name()))
                         ((PerColumnSecondaryIndex) index).insert(key, column);
                 }
             }