add null-pointer check

lib/hash.c

@@ -45,6 +45,7 @@ hash_element_dtor(void *user, void *element)
   if(e->key)
     free(e->key);
 
+  if(e->ptr)
     h->dtor(e->ptr);
 
   free(e);

lib/hostip6.c

@@ -85,6 +85,7 @@
  */
 void Curl_freeaddrinfo(Curl_addrinfo *p)
 {
+  if(p)
     freeaddrinfo(p);
 }