url: make Curl_close() NULLify the pointer too

This is the common pattern used in the code and by a unified approach we
avoid mistakes.

Closes #4534

lib/easy.c

@@ -731,7 +731,7 @@ void curl_easy_cleanup(struct Curl_easy *data)
     return;
 
   sigpipe_ignore(data, &pipe_st);
-  Curl_close(data);
+  Curl_close(&data);
   sigpipe_restore(&pipe_st);
 }