sstablex/mx/reader: validate(): reset the current PI block on partition-start

It is possible that the next partition has no PI and thus there won't be
a new PI block to overwrite the old one. This will result in
false-positive messages about rows being outside of the finished PI
block.

(cherry picked from commit e073df1dbb)

sstables/mx/reader.cc

@@ -1952,6 +1952,7 @@ public:
             report_error(res.what());
             _validator.reset(dk);
         }
+        _expected_clustering_block.reset();
         if (_stop_after_partition_header) {
             _stop_after_partition_header = false;
             return data_consumer::proceed::no;