Fix integer overflow when calculating zeroing range in zeroObject

Lists can have up to 29 bits of elements. We multiply that by a constant of 8 (Bytes per word), so basically any element size can cause on overflow.

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

@@ -258,7 +258,7 @@ final class WireHelpers {
             case ElementSize.EIGHT_BYTES: {
                 memset(segment.buffer, ptr * Constants.BYTES_PER_WORD, (byte) 0,
                         roundBitsUpToWords(
-                           ListPointer.elementCount(tag) *
+                                (long) ListPointer.elementCount(tag) *
                                         ElementSize.dataBitsPerElement(ListPointer.elementSize(tag))) * Constants.BYTES_PER_WORD);
                 break;
             }