[v3.7] Crash if enabling v4-mapping-to-v6 and DHCPv4

[jukkar]

If user enables CONFIG_NET_IPV4_MAPPING_TO_IPV6 and CONFIG_NET_DHCPV4 at the same time, then net_dhcpv4_init() will call net_udp_register() with NULL net_context pointer. This in turn will cause a NULL pointer access when checking V6ONLY socket option.