dhcp-client: reject messages larger than the maximum UDP payload

dhcp-client: reject messages larger than the maximum UDP payload

dhcp_message_verify_header() only enforced a lower bound on the input
length, so dhcp_message_parse() happily accepted arbitrarily large
buffers. Such inputs could never have been received via UDP and would
later fail in dhcp_message_build() with -E2BIG once the parsed options'
combined size exceeds UDP_PAYLOAD_MAX_SIZE, which the fuzzer surfaced as
an assertion failure.

Reject inputs above UDP_PAYLOAD_MAX_SIZE up front, so the parse stage
mirrors what the wire format can actually carry.

Follow-up for 8c18bb6547c2138f2f17b921ec06f2c1f7cd17cd

Fixes https://github.com/systemd/systemd/issues/42147

Co-developed-by: Claude Opus 4.7 <noreply@anthropic.com>