networkd: fix for networkd crash when client sends Option 82 via SendOption=

networkd: fix for networkd crash when client sends Option 82 via SendOption=

When a DHCP client uses SendOption=82:string:..., option_append() calls
the SD_DHCP_OPTION_RELAY_AGENT_INFORMATION case which was written for
the server relay path. It casts optval to sd_dhcp_server* and calls
strlen() on its members, but optval is actually raw binary data from
the client, causing SIGSEGV. The same is applicable when option 43 and
option 77 are passed to SendOption.

Fix by checking optlen > 0 and appending the option as a plain TLV,
skipping the server-specific relay agent logic.