Initialized pinned_pos_ and pinned_seq_pos_ in FragmentedRangeTombstoneIterator (#5720)

Summary:
These uninitialized member variables can cause a key to not be pinned when it should be, causing erroneous behavior. For example ingesting a file with range deletion tombstones will yield an "external file have corrupted keys" on a Mac.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5720

Differential Revision: D17217673

fbshipit-source-id: cd7df7ce3ad9cf69c841c4d3dc6fd144eff9e212

db/range_tombstone_fragmenter.h

@@ -144,6 +144,8 @@ class FragmentedRangeTombstoneIterator : public InternalIterator {
   void Invalidate() {
     pos_ = tombstones_->end();
     seq_pos_ = tombstones_->seq_end();
+    pinned_pos_ = tombstones_->end();
+    pinned_seq_pos_ = tombstones_->seq_end();
   }
 
   RangeTombstone Tombstone() const {