Fix for partially read ultralight tag

[benhowes]

In my use case, I found it relatively common to be able to retrieve partial NDEF messages, for example a tag with a URI ndef record of https://example.com, would often be returned as http!I��⸮�⸮��⸮⸮���⸮���⸮, due to reading uninitialized memory.

It seems to happen due to:

1. The MifareUltralight::read calls the NdefMessage constructor regardless of if the messageLength is 0
2. Line 20 of the NdefMessage::NdefMessage constructor contains while (index <= numBytes) - which does allow a partial NDEF message to be processed, even if numBytes is 0.

My fix is to not call the NdefMessage constructor if there was an error reading the message.

It seems to make corrupted reads a thing of the past for me!