fix bug in short to int conversion in StructPointer

runtime/src/main/java/org/capnproto/StructPointer.java

@@ -34,7 +34,7 @@ final class StructPointer{
     }
 
     public static int wordSize(long ref) {
-        return (int)dataSize(ref) + (int)ptrCount(ref);
+        return Short.toUnsignedInt(dataSize(ref)) + Short.toUnsignedInt(ptrCount(ref));
     }
 
     public static void setFromStructSize(ByteBuffer buffer, int offset, StructSize size) {

runtime/src/test/java/org/capnproto/StructPointerTest.java

@@ -0,0 +1,21 @@
+package org.capnproto;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+public class StructPointerTest {
+
+    @Test
+    public void testWordSize() {
+        Assert.assertEquals(
+            3,
+            StructPointer.wordSize(0x0001000200000000L));
+    }
+
+    @Test
+    public void testWordSizeUnderflow() {
+        Assert.assertEquals(
+            0x1fffe,
+            StructPointer.wordSize(0xffffffff00000000L));
+    }
+}