BUG/MEDIUM: connection: sanitize PPv2 header length before parsing address information

BUG/MEDIUM: connection: sanitize PPv2 header length before parsing address information

Previously, if hdr_v2->len was less than the length of the protocol
specific address information we could have read after the end of the
buffer and initialize the sockaddr structure with junk.

Signed-off-by: KOVACS Krisztian <hidden@balabit.com>
[WT: this is only tagged medium since proxy protocol is only used from
 trusted sources]

This must be backported to 1.5.