Fix IndexOutOfBoundsException for CompositeByteBuf netty#4679

Motivation:

Modifications:

Use the correct start index

Result:

Fixes netty#4679

Author: windie

buffer/src/main/java/io/netty/buffer/ByteBufUtil.java

@@ -548,7 +548,7 @@ static String decodeString(ByteBuf src, int readerIndex, int len, Charset charse
             try {
                 buffer.writeBytes(src, readerIndex, len);
                 // Use internalNioBuffer(...) to reduce object creation.
-                decodeString(decoder, buffer.internalNioBuffer(readerIndex, len), dst);
+                decodeString(decoder, buffer.internalNioBuffer(0, len), dst);
             } finally {
                 // Release the temporary buffer again.
                 buffer.release();

buffer/src/test/java/io/netty/buffer/ByteBufUtilTest.java

@@ -187,4 +187,17 @@ private static void testDecodeString(String text, Charset charset) {
         Assert.assertEquals(text, ByteBufUtil.decodeString(buffer, 0, buffer.readableBytes(), charset));
         buffer.release();
     }
+
+    @Test
+    public void testToStringDoesNotThrowIndexOutOfBounds() {
+        CompositeByteBuf buffer = Unpooled.compositeBuffer();
+        try {
+            byte[] bytes = "1234".getBytes(CharsetUtil.UTF_8);
+            buffer.addComponent(Unpooled.buffer(bytes.length).writeBytes(bytes));
+            buffer.addComponent(Unpooled.buffer(bytes.length).writeBytes(bytes));
+            Assert.assertEquals("1234", buffer.toString(bytes.length, bytes.length, CharsetUtil.UTF_8));
+        } finally {
+            buffer.release();
+        }
+    }
 }