Fixed bitcoinj DoS. It could have been crashed by a malicious message.

Bitcoinj can be crashed with OutOfMemory by sending a message with
a large claimed var_str length or bytes array length.

The actual message size does not matter, it's the claimed length that matters.

This affects all bitcoinj-based apps that receive messages including Multibit, Android Bitcoin Wallet, Mycelium and Hive.

The fix limits accepted length to max message size (32 MB).

Signed-off-by: Mike Hearn <mike@plan99.net>