stop populating unused/invalid MergingIterator heaps (#8975)

Summary:
I was looking at https://github.com/facebook/rocksdb/issues/2636 and got very confused that `MergingIterator::AddIterator()` is populating `min_heap_` with dangling pointers. There is justification in the comments that `min_heap_` will be cleared before it's used, but it'd be cleaner to not populate it with dangling pointers in the first place. Also made similar change in the constructor for consistency, although the pointers there would not be dangling, just unused.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8975

Test Plan: rely on existing tests

Reviewed By: pdillinger, hx235

Differential Revision: D31273767

Pulled By: ajkr

fbshipit-source-id: 127ca9dd1f82f77f55dd0c3f19511de3282fc229

table/merging_iterator.cc

@@ -50,10 +50,6 @@ class MergingIterator : public InternalIterator {
     for (int i = 0; i < n; i++) {
       children_[i].Set(children[i]);
     }
-    for (auto& child : children_) {
-      AddToMinHeapOrCheckStatus(&child);
-    }
-    current_ = CurrentForward();
   }
 
   void considerStatus(Status s) {
@@ -63,16 +59,13 @@ class MergingIterator : public InternalIterator {
   }
 
   virtual void AddIterator(InternalIterator* iter) {
-    assert(direction_ == kForward);
     children_.emplace_back(iter);
     if (pinned_iters_mgr_) {
       iter->SetPinnedItersMgr(pinned_iters_mgr_);
     }
-    auto new_wrapper = children_.back();
-    AddToMinHeapOrCheckStatus(&new_wrapper);
-    if (new_wrapper.Valid()) {
-      current_ = CurrentForward();
-    }
+    // Invalidate to ensure `Seek*()` is called to construct the heaps before
+    // use.
+    current_ = nullptr;
   }
 
   ~MergingIterator() override {