free: instead of Curl_safefree()

Since we just started make use of free(NULL) in order to simplify code,
this change takes it a step further and:

- converts lots of Curl_safefree() calls to good old free()
- makes Curl_safefree() not check the pointer before free()

The (new) rule of thumb is: if you really want a function call that
frees a pointer and then assigns it to NULL, then use Curl_safefree().
But we will prefer just using free() from now on.

lib/asyn-thread.c

@@ -393,7 +393,7 @@ static bool init_resolve_thread (struct connectdata *conn,
   if(!init_thread_sync_data(td, hostname, port, hints))
     goto err_exit;
 
-  Curl_safefree(conn->async.hostname);
+  free(conn->async.hostname);
   conn->async.hostname = strdup(hostname);
   if(!conn->async.hostname)
     goto err_exit;