Fix assertion message in DefaultDataBuffer

Closes gh-27567

spring-core/src/main/java/org/springframework/core/io/buffer/DefaultDataBuffer.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2020 the original author or authors.
+ * Copyright 2002-2021 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -462,9 +462,9 @@ public class DefaultDataBuffer implements DataBuffer {
 
 	private void checkIndex(int index, int length) {
 		assertIndex(index >= 0, "index %d must be >= 0", index);
-		assertIndex(length >= 0, "length %d must be >= 0", index);
+		assertIndex(length >= 0, "length %d must be >= 0", length);
 		assertIndex(index <= this.capacity, "index %d must be <= %d", index, this.capacity);
-		assertIndex(length <= this.capacity, "length %d must be <= %d", index, this.capacity);
+		assertIndex(length <= this.capacity, "length %d must be <= %d", length, this.capacity);
 	}
 
 	private void assertIndex(boolean expression, String format, Object... args) {