Checking ifaddr pointer for NULL without checking in6_ifaddr works

Checking ifaddr pointer for NULL without checking in6_ifaddr works
as ifaddr ia_ifa is the first field of in6_ifaddr.  So the pointers
are the same, and one NULL check works for both.  But in ISO C NULL
has some kind of type and this is undefined behavior.  So add a
second NULL check that the compiler can optimize away.  The resulting
assembler is the same.
found by kubsan; OK tobhe@