Compute a tighter upper bound on the buffer needed for packed encoding

src/node-capnp/capnp.cc

@@ -1778,7 +1778,9 @@ v8::Handle<v8::Value> toBytesPacked(const v8::Arguments& args) {
 
   return liftKj([&]() -> v8::Handle<v8::Value> {
     auto unpackedSize = computeSerializedSizeInWords(builder.message) * sizeof(capnp::word);
-    auto packedSizeUpperBound = unpackedSize * 258 / 256;
+    // Packed encoding can add 2 bytes of overhead per 2k segment reached.
+    auto packedOverhead = (((unpackedSize - 1) / 2048) + 1) * 2;
+    auto packedSizeUpperBound = unpackedSize + packedOverhead;
     auto oversizedBuf = kj::heapArray<byte>(packedSizeUpperBound);
     kj::ArrayOutputStream outputStream(oversizedBuf);
     capnp::writePackedMessage(outputStream, builder.message);