Fix HttpPostMultipartRequestDecoder.splitMultipartHeader() String index out of range: -1 with empty header

Motivation:

A Malformed empty header value (e.g. Content-Type: \r\n) will trigger a String index out of range
while trying to parse the multi-part request, using the HttpPostMultipartRequestDecoder.

Modification:

Ensure that the substring() method is called passing the endValue >= valueStart.
In case of an empty header value, the empty header value associated with the header key will be returned.

Result:

Fixes #7620

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

@@ -1467,7 +1467,7 @@ public class HttpPostMultipartRequestDecoder implements InterfaceHttpPostRequest
         valueStart = HttpPostBodyUtil.findNonWhitespace(sb, colonEnd);
         valueEnd = HttpPostBodyUtil.findEndOfString(sb);
         headers.add(sb.substring(nameStart, nameEnd));
-        String svalue = sb.substring(valueStart, valueEnd);
+        String svalue = (valueStart >= valueEnd) ? StringUtil.EMPTY_STRING : sb.substring(valueStart, valueEnd);
         String[] values;
         if (svalue.indexOf(';') >= 0) {
             values = splitMultipartHeaderValues(svalue);

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

@@ -626,4 +626,34 @@ public class HttpPostRequestDecoderTest {
             req.release();
         }
     }
+
+    // https://github.com/netty/netty/issues/7620
+    @Test
+    public void testDecodeMalformedEmptyContentTypeFieldParameters() throws Exception {
+        final String boundary = "dLV9Wyq26L_-JQxk6ferf-RT153LhOO";
+        final DefaultFullHttpRequest req = new DefaultFullHttpRequest(HttpVersion.HTTP_1_1, HttpMethod.POST,
+                                                                      "http://localhost");
+        req.headers().add(HttpHeaderNames.CONTENT_TYPE, "multipart/form-data; boundary=" + boundary);
+        // Force to use memory-based data.
+        final DefaultHttpDataFactory inMemoryFactory = new DefaultHttpDataFactory(false);
+        final String data = "asdf";
+        final String filename = "tmp-0.txt";
+        final String body =
+                "--" + boundary + "\r\n" +
+                "Content-Disposition: form-data; name=\"file\"; filename=\"" + filename + "\"\r\n" +
+                "Content-Type: \r\n" +
+                "\r\n" +
+                data + "\r\n" +
+                "--" + boundary + "--\r\n";
+
+        req.content().writeBytes(body.getBytes(CharsetUtil.UTF_8.name()));
+        // Create decoder instance to test.
+        final HttpPostRequestDecoder decoder = new HttpPostRequestDecoder(inMemoryFactory, req);
+        assertFalse(decoder.getBodyHttpDatas().isEmpty());
+        InterfaceHttpData part1 = decoder.getBodyHttpDatas().get(0);
+        assertTrue(part1 instanceof FileUpload);
+        FileUpload fileUpload = (FileUpload) part1;
+        assertEquals("tmp-0.txt", fileUpload.getFilename());
+        decoder.destroy();
+    }
 }