Fix TestPostingsUtil#testIntegerOverflow failure. (#13979)

The group vint logic is mistakenly using the long->int conversion logic for the
case when integers are being written rather than longs.

Closes #13978

lucene/core/src/java/org/apache/lucene/util/GroupVIntUtil.java

@@ -292,7 +292,7 @@ public static void writeGroupVInts(DataOutput out, byte[] scratch, int[] values,
 
     // tail vints
     for (; readPos < limit; readPos++) {
-      out.writeVInt(toInt(values[readPos]));
+      out.writeVInt(values[readPos]);
     }
   }
 }

lucene/core/src/test/org/apache/lucene/codecs/lucene101/TestPostingsUtil.java

@@ -27,7 +27,15 @@ public class TestPostingsUtil extends LuceneTestCase {
 
   // checks for bug described in https://github.com/apache/lucene/issues/13373
   public void testIntegerOverflow() throws IOException {
-    final int size = random().nextInt(1, ForUtil.BLOCK_SIZE);
+    // Size that writes the first value as a regular vint
+    int randomSize1 = random().nextInt(1, 3);
+    // Size that writes the first value as a group vint
+    int randomSize2 = random().nextInt(4, ForUtil.BLOCK_SIZE);
+    doTestIntegerOverflow(randomSize1);
+    doTestIntegerOverflow(randomSize2);
+  }
+
+  private void doTestIntegerOverflow(int size) throws IOException {
     final int[] docDeltaBuffer = new int[size];
     final int[] freqBuffer = new int[size];