Fix IAE on cross_fields query introduced in 7.0.1 (#41938)

If the max doc in the index is greater than the minimum total term frequency
among the requested fields we need to adjust max doc to be equal to the min ttf.
This was removed by mistake when fixing #41125.

Closes #41934

server/src/main/java/org/apache/lucene/queries/BlendedTermQuery.java

@@ -118,7 +118,9 @@ public abstract class BlendedTermQuery extends Query {
                 // otherwise the statistics don't match
                 minSumTTF = Math.min(minSumTTF, reader.getSumTotalTermFreq(terms[i].field()));
             }
-
+        }
+        if (maxDoc > minSumTTF) {
+            maxDoc = (int)minSumTTF;
         }
         if (max == 0) {
             return; // we are done that term doesn't exist at all

server/src/test/java/org/apache/lucene/queries/BlendedTermQueryTests.java

@@ -227,4 +227,34 @@ public class BlendedTermQueryTests extends ESTestCase {
         assertThat(extracted.size(), equalTo(terms.size()));
         assertThat(extracted, containsInAnyOrder(terms.toArray(new Term[0])));
     }
+
+    public void testMinTTF() throws IOException {
+        Directory dir = newDirectory();
+        IndexWriter w = new IndexWriter(dir, newIndexWriterConfig(new MockAnalyzer(random())));
+        FieldType ft = new FieldType(TextField.TYPE_NOT_STORED);
+        ft.freeze();
+
+        for (int i = 0; i < 10; i++) {
+            Document d = new Document();
+            d.add(new TextField("id", Integer.toString(i), Field.Store.YES));
+            d.add(new Field("dense", "foo foo foo", ft));
+            if (i % 10 == 0) {
+                d.add(new Field("sparse", "foo", ft));
+            }
+            w.addDocument(d);
+        }
+        w.commit();
+        DirectoryReader reader = DirectoryReader.open(w);
+        IndexSearcher searcher = setSimilarity(newSearcher(reader));
+        {
+            String[] fields = new String[]{"dense", "sparse"};
+            Query query = BlendedTermQuery.dismaxBlendedQuery(toTerms(fields, "foo"), 0.1f);
+            TopDocs search = searcher.search(query, 10);
+            ScoreDoc[] scoreDocs = search.scoreDocs;
+            assertEquals(Integer.toString(0), reader.document(scoreDocs[0].doc).getField("id").stringValue());
+        }
+        reader.close();
+        w.close();
+        dir.close();
+    }
 }