The UDP proxy corrupts application data at the end of the datagram. If
there are multiple DTLS records within the same datagram, this leads
to the wrong message being corrupted. This commit always corrupts the
beginning of the message to prevent this.
Overall, the UDP proxy needs reworking if it is supposed to reliably
support multiple records within a single datagram, because it
determines its actions from the type of the first record in the
current datagram only.
fbb0b701e4