ares_getnameinfo(): loosen validation on salen

salen validation should be greater than or equal to the required
storage size.  Its not uncommon to use `struct sockaddr_storage` in
modern code which is definitely larger than `struct sockaddr_in` and
on some systems even larger than `struct sockaddr_in6`.

Fixes Issue: #752
Fix By: Brad House (@bradh352)

src/lib/ares_getnameinfo.c

@@ -98,11 +98,12 @@ static void  ares_getnameinfo_int(ares_channel_t        *channel,
   unsigned int               flags = (unsigned int)flags_int;
 
   /* Validate socket address family and length */
-  if ((sa->sa_family == AF_INET) && (salen == sizeof(struct sockaddr_in))) {
+  if (sa && sa->sa_family == AF_INET &&
+      salen >= sizeof(struct sockaddr_in)) {
     addr = CARES_INADDR_CAST(struct sockaddr_in *, sa);
     port = addr->sin_port;
-  } else if ((sa->sa_family == AF_INET6) &&
-             (salen == sizeof(struct sockaddr_in6))) {
+  } else if (sa && sa->sa_family == AF_INET6 &&
+             salen >= sizeof(struct sockaddr_in6)) {
     addr6 = CARES_INADDR_CAST(struct sockaddr_in6 *, sa);
     port  = addr6->sin6_port;
   } else {