util: allow calling virSocketAddrGetIPPrefix with NULL netmask or address

util: allow calling virSocketAddrGetIPPrefix with NULL netmask or address

There are times when we don't have a netmask pointer to give to
virSocketAddrGetIPPrefix() (e.g. the IP addresses in domain interfaces
only have a prefix, no netmask), but it would have caused a segv if we
called it with NULL instead of a pointer to a netmask. This patch
qualifies the code that would use the netmask or address pointers to
check for NULL first.