[Java] Flexbuffers - Negative signed object length (#6651)

* retry with correct email

* update JavaTest.java

java/com/google/flatbuffers/FlexBuffers.java

@@ -655,7 +655,7 @@ public class FlexBuffers {
 
         Sized(ReadBuf buff, int end, int byteWidth) {
             super(buff, end, byteWidth);
-            size = readInt(bb, end - byteWidth, byteWidth);
+            size = (int) readUInt(bb, end - byteWidth, byteWidth);
         }
 
         public int size() {

tests/JavaTest.java

@@ -862,6 +862,27 @@ class JavaTest {
         TestEq((byte)-1, result[3]);
     }
 
+    public static void testSingleElementLongBlob() {
+
+        // verifies blobs of up to 2^16 in length
+        for (int i = 2; i <= 1<<16; i = i<<1) {
+            byte[] input = new byte[i-1];
+            for (int index = 0; index < input.length; index++) {
+                input[index] = (byte)(index % 64);
+            }
+
+            FlexBuffersBuilder builder = new FlexBuffersBuilder();
+            builder.putBlob(input);
+            ByteBuffer b = builder.finish();
+            FlexBuffers.Reference r = FlexBuffers.getRoot(b);
+            byte[] result = r.asBlob().getBytes();
+            
+            for (int index = 0; index < input.length; index++) {
+                TestEq((byte)(index % 64), result[index]);
+            }
+        }
+    }
+
     public static void testSingleElementUByte() {
         FlexBuffersBuilder builder = new FlexBuffersBuilder();
         builder.putUInt(0xFF);
@@ -1084,6 +1105,7 @@ class JavaTest {
         testSingleElementSmallString();
         testSingleElementBigString();
         testSingleElementBlob();
+        testSingleElementLongBlob();
         testSingleElementVector();
         testSingleFixedTypeVector();
         testSingleElementUShort();