net: dhcpv6: Verify net_pkt_skip() return value

Verify the return value of net_pkt_skip() function, in case the parser
ignores the unrecognized options, so that in case the option was
malformed and the actual provided option length exceeds the packet
length, it is recognize (net_pkt_skip() should fail then).

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>