Skip range deletions at seqno zero when collapsing (#4216)

Summary:
`CollapsedRangeDelMap` internally uses seqno zero as a sentinel value to
denote a gap between range tombstones or the end of range tombstones. It
therefore expects to never have consecutive sentinel tombstones.

However, since `DeleteRange` is now supported in `SstFileWriter`, an
ingested file may contain range tombstones, and that ingested file may
be assigned global seqno zero. When such tombstones are added to the
collapsed map, they resemble sentinel tombstones due to having seqno
zero. Then, the invariant mentioned above about never having consecutive
sentinel tombstones can be violated.

The symptom of this violation was dereferencing the `end()` iterator
(#4204). The fix in this PR is to not add range tombstones with seqno
zero to the collapsed map. They're not needed anyways since they can't
possibly cover anything (in case of a key and a range tombstone with the
same seqno, the key is visible).
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4216

Differential Revision: D9121716

Pulled By: ajkr

fbshipit-source-id: f5b78a70bea9527354603ea7ac8542a7e2b6a210

db/range_del_aggregator.cc

@@ -155,6 +155,9 @@ class CollapsedRangeDelMap : public RangeDelMap {
     }
 
     RangeTombstone Tombstone() const override {
+      assert(Valid());
+      assert(std::next(iter_) != rep_.end());
+      assert(iter_->second != 0);
       RangeTombstone tombstone;
       tombstone.start_key_ = iter_->first;
       tombstone.end_key_ = std::next(iter_)->first;
@@ -232,7 +235,8 @@ class CollapsedRangeDelMap : public RangeDelMap {
   }
 
   void AddTombstone(RangeTombstone t) {
-    if (ucmp_->Compare(t.start_key_, t.end_key_) >= 0) {
+    if (ucmp_->Compare(t.start_key_, t.end_key_) >= 0 ||
+        t.seq_ == 0) {
       // The tombstone covers no keys. Nothing to do.
       return;
     }