src: fix bad sizeof expression

It was computing the size of the pointer, not the size of the pointed-to
object.

Introduced in commit 727b291 ("src,dns: refactor cares_wrap to avoid
global state".)

PR-URL: #17014
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>

src/cares_wrap.cc

@@ -158,7 +158,7 @@ class ChannelWrap : public AsyncWrap {
   }
   inline node_ares_task_list* task_list() { return &task_list_; }
 
-  size_t self_size() const override { return sizeof(this); }
+  size_t self_size() const override { return sizeof(*this); }
 
   static void AresTimeout(uv_timer_t* handle);