LUCENE-8568: TestMockDirectoryWrapper/ RAMInputStream NPE.

lucene/core/src/java/org/apache/lucene/store/RAMInputStream.java

@@ -72,17 +72,24 @@ public class RAMInputStream extends IndexInput implements Cloneable {
     if (bufferPosition == bufferLength) {
       nextBuffer();
     }
-    return currentBuffer[bufferPosition++];
+    if (currentBuffer == null) {
+      throw new EOFException();
+    } else {
+      return currentBuffer[bufferPosition++];
+    }
   }
 
   @Override
   public void readBytes(byte[] b, int offset, int len) throws IOException {
     while (len > 0) {
-
       if (bufferPosition == bufferLength) {
         nextBuffer();
       }
 
+      if (currentBuffer == null) {
+        throw new EOFException();
+      }
+
       int remainInBuffer = bufferLength - bufferPosition;
       int bytesToCopy = len < remainInBuffer ? len : remainInBuffer;
       System.arraycopy(currentBuffer, bufferPosition, b, offset, bytesToCopy);

lucene/test-framework/src/java/org/apache/lucene/store/BaseDirectoryTestCase.java

@@ -522,6 +522,9 @@ public abstract class BaseDirectoryTestCase extends LuceneTestCase {
 
       // But any read following the seek(len) should throw an EOFException.
       expectThrows(EOFException.class, i::readByte);
+      expectThrows(EOFException.class, () -> {
+        i.readBytes(new byte [1], 0, 1);
+      });
 
       i.close();
     }