ipv4.c uses a gateway address of INADDR_NONE to represent "no
gateway". It initialises the gateway address to INADDR_NONE before
calling fetch_ipv4_setting() to retrieve the configured gateway
address (if any).
However, as of commit 612f4e7 "[settings] Avoid returning
uninitialised data on error in fetch_xxx_setting()",
fetch_ipv4_setting() will zero the IP address if the setting does not
exist, rather than leaving it unaltered.
Fix by using a zero IP address to indicate "no gateway", so that a
non-existent gateway address setting will be treated as such.
