report: do not use uv_default_loop() as fallback

Not seeing an associated `Environment` is a rare condition anyway,
but using `uv_default_loop()` as a fallback is not thread-safe.

Author: addaleax

src/node_report.cc

@@ -307,8 +307,6 @@ static void WriteNodeReport(Isolate* isolate,
   writer.json_arraystart("libuv");
   if (env != nullptr)
     uv_walk(env->event_loop(), WalkHandle, static_cast<void*>(&writer));
-  else
-    uv_walk(uv_default_loop(), WalkHandle, static_cast<void*>(&writer));
 
   writer.json_arrayend();