LUCENE-7887: don't clear BufferedUpdatesStream on abort

lucene/core/src/java/org/apache/lucene/index/BufferedUpdatesStream.java

@@ -175,11 +175,14 @@ class BufferedUpdatesStream implements Accountable {
     // TODO: would be a bit more memory efficient to track this per-segment, so when each segment writes it writes all packets finished for
     // it, rather than only recording here, across all segments.  But, more complex code, and more CPU, and maybe not so much impact in
     // practice?
+    assert packet.applied.getCount() == 1: "packet=" + packet;
 
     packet.applied.countDown();
 
     updates.remove(packet);
     numTerms.addAndGet(-packet.numTermDeletes);
+    assert numTerms.get() >= 0: "numTerms=" + numTerms + " packet=" + packet;
+    
     bytesUsed.addAndGet(-packet.bytesUsed);
 
     finishedSegment(packet.delGen());

lucene/core/src/java/org/apache/lucene/index/FrozenBufferedUpdates.java

@@ -299,7 +299,6 @@ class FrozenBufferedUpdates {
           if (infoStream.isEnabled("BD")) {
             infoStream.message("BD", "packet matches no segments");
           }
-          
           break;
         }
 
@@ -422,10 +421,7 @@ class FrozenBufferedUpdates {
       throw new IllegalArgumentException("gen is not yet set; call BufferedUpdatesStream.push first");
     }
 
-    if (applied.getCount() == 0) {
-      // already done
-      return totalDelCount;
-    }
+    assert applied.getCount() != 0;
 
     if (privateSegment != null) {
       assert segStates.length == 1;

lucene/core/src/java/org/apache/lucene/index/IndexWriter.java

@@ -2364,8 +2364,6 @@ public class IndexWriter implements Closeable, TwoPhaseCommit, Accountable {
       // set it to false before calling rollbackInternal
       mergeScheduler.close();
 
-      bufferedUpdatesStream.clear();
-
       docWriter.close(); // mark it as closed first to prevent subsequent indexing actions/flushes 
       docWriter.abort(this); // don't sync on IW here
       synchronized(this) {