Fix use of deleted file iterators with incomplete iterators

Summary:
After deleting file iterators which are over the iterate upper
bound, we also need to check for null pointers in
ResetIncompletIterators.

Test Plan: db_tailing_iter_test.TailingIteratorTrimSeekToNext

Reviewers: tnovak, sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D45525

db/db_tailing_iter_test.cc

@@ -171,6 +171,15 @@ TEST_F(DBTestTailingIterator, TailingIteratorTrimSeekToNext) {
     ASSERT_TRUE(itern->Valid());
     ASSERT_EQ(itern->key().compare(key), 0);
   }
+
+  read_options.read_tier = kBlockCacheTier;
+  std::unique_ptr<Iterator> iteri(db_->NewIterator(read_options, handles_[1]));
+  char buf5[32];
+  snprintf(buf5, sizeof(buf5), "00a0%016d", (num_records / 2) * 5 - 2);
+  Slice target1(buf5, 20);
+  iteri->Seek(target1);
+  ASSERT_TRUE(iteri->Valid() || iteri->status().IsIncomplete());
+
   for (int i = 2 * num_records; i > 0; --i) {
     char buf1[32];
     char buf2[32];

db/forward_iterator.cc

@@ -514,7 +514,7 @@ void ForwardIterator::ResetIncompleteIterators() {
   const auto& l0_files = sv_->current->storage_info()->LevelFiles(0);
   for (uint32_t i = 0; i < l0_iters_.size(); ++i) {
     assert(i < l0_files.size());
-    if (!l0_iters_[i]->status().IsIncomplete()) {
+    if (!l0_iters_[i] || !l0_iters_[i]->status().IsIncomplete()) {
       continue;
     }
     delete l0_iters_[i];