Avoid a null dereference if IFF_BROADCAST is set but there is no broadcast address

It seems that the getifaddrs() function can return interfaces with
IFF_BROADCAST & IFF_UP set, but no broadcast address (at least
under Cygwin 1.7, this seems to happen for v6 mapped v4 addresses)

Avoid a null dereference if this ever happens

Signed-off-by: Jon TURNEY <jon.turney@dronecode.org.uk>
Reviewed-by: Colin Harrison <colin.harrison@virgin.net>

os/access.c

@@ -811,7 +811,8 @@ DefineSelf (int fd)
 		continue;
 #endif
 	    if ((ifr->ifa_flags & IFF_BROADCAST) &&
-		(ifr->ifa_flags & IFF_UP))
+		(ifr->ifa_flags & IFF_UP) &&
+                ifr->ifa_broadaddr)
 		broad_addr = *ifr->ifa_broadaddr;
 	    else
 		continue;