Return null in HttpPostRequestEncoder (#9352)

Motivation:

If the encoded value of a form element happens to exactly hit
the chunk limit (8096 bytes), the post request encoder will
throw a NullPointerException.

Modifications:

Catch the null case and return.

Result:

No NPE.

codec-http/src/main/java/io/netty/handler/codec/http/multipart/HttpPostRequestEncoder.java

@@ -975,7 +975,11 @@ public class HttpPostRequestEncoder implements ChunkedInput<HttpContent> {
         if (buffer.capacity() == 0) {
             currentData = null;
             if (currentBuffer == null) {
-                currentBuffer = delimiter;
+                if (delimiter == null) {
+                    return null;
+                } else {
+                    currentBuffer = delimiter;
+                }
             } else {
                 if (delimiter != null) {
                     currentBuffer = wrappedBuffer(currentBuffer, delimiter);

codec-http/src/test/java/io/netty/handler/codec/http/multipart/HttpPostRequestEncoderTest.java

@@ -18,10 +18,12 @@ package io.netty.handler.codec.http.multipart;
 import io.netty.buffer.ByteBuf;
 import io.netty.buffer.ByteBufAllocator;
 import io.netty.buffer.Unpooled;
+import io.netty.handler.codec.http.DefaultHttpRequest;
 import io.netty.handler.codec.http.DefaultFullHttpRequest;
 import io.netty.handler.codec.http.HttpConstants;
 import io.netty.handler.codec.http.HttpContent;
 import io.netty.handler.codec.http.HttpMethod;
+import io.netty.handler.codec.http.HttpRequest;
 import io.netty.handler.codec.http.HttpVersion;
 import io.netty.handler.codec.http.LastHttpContent;
 import io.netty.handler.codec.http.multipart.HttpPostRequestEncoder.EncoderMode;
@@ -443,4 +445,27 @@ public class HttpPostRequestEncoderTest {
                 + readable, expectedSize);
         httpContent.release();
     }
+
+    @Test
+    public void testEncodeChunkedContent() throws Exception {
+        HttpRequest req = new DefaultHttpRequest(HttpVersion.HTTP_1_1, HttpMethod.POST, "/");
+        HttpPostRequestEncoder encoder = new HttpPostRequestEncoder(req, false);
+
+        int length = 8077 + 8096;
+        char[] array = new char[length];
+        Arrays.fill(array, 'a');
+        String longText = new String(array);
+
+        encoder.addBodyAttribute("data", longText);
+        encoder.addBodyAttribute("moreData", "abcd");
+
+        assertNotNull(encoder.finalizeRequest());
+
+        while (!encoder.isEndOfInput()) {
+            encoder.readChunk((ByteBufAllocator) null).release();
+        }
+
+        assertTrue(encoder.isEndOfInput());
+        encoder.cleanFiles();
+    }
 }