Ignore invalid base64 messages
Ignore invalid base64 messages

When base64 encoding is enabled, ignore any messages that fail base64
decoding.

Previously signed-unsigned integer conversion would cause the return
value of b64_pton() on error (a negative integer) to be converted to a
large value. The attempt to allocate this value would force xmppipe to
exit.

msantos/xmppipe,msantos/xmppipe