On my linux system, getifaddrs() sometimes returns network interfaces with
ifa_addr = 0.  This causes DefineSelf() in os/access.c to call ConvertAddress
with NULL as its first argument, which then causes a segmentation fault.

The interface in question is a ppp interface with no local IP configured, but
I'm not sure whether this is a Linux/glibc bug or an Xorg one.

Anyway, adding the obvious check (if (ifr->ifa_addr == NULL) continue; in the
for loop at access.c:978) fixes the crash.  I believe that should be included as
a sanity check even if there is a standard that says getifaddrs() shouldn't
return such data.