tinc 1.1: freeaddrinfo(NULL) crash on windows

[Todd C. Miller]

On Windows, freeaddrinfo(NULL) will result in a segv.  In
get_recent_address(), there is the following block of code:

        if(cache->aip) {
                sockaddr_t *sa = (sockaddr_t *)cache->aip->ai_addr;
                cache->aip = cache->aip->ai_next;

                if(!cache->aip) {
                        freeaddrinfo(cache->aip);
                        cache->aip = NULL;
                }

                return sa;
        }

where freeaddrinfo() is called when cache->aip is guaranteed to be
NULL.  I get a bit confused with respect to cache->ai vs. cache->aip
but that part of the code looks suspicious.

At first I thought it was intended to free the old value of cache->aip
but since sa points to memory within that chunk, freeing it would
result in a use after free situation.

 - todd