http2: improved on_stream_close/data_done handling

- there seems to be a code path that cleans up easy handles without
  triggering DONE or DETACH events to the connection filters. This
  would explain wh nghttp2 still holds stream user data
- add GOOD check to easy handle used in on_close_callback to
  prevent crashes, ASSERTs in debug builds.
- NULL the stream user data early before submitting RST
- add checks in on_stream_close() to identify UNGOOD easy handles

Reported-by: Hans-Christian Egtvedt
Fixes #10936
Closes #12562

tests/http/testenv/curl.py

@@ -247,7 +247,7 @@ class ExecResult:
         if exitcode is not None:
             for idx, x in enumerate(self.stats):
                 if 'exitcode' in x:
-                    assert x['exitcode'] == 0, \
+                    assert x['exitcode'] == exitcode, \
                         f'status #{idx} exitcode: expected {exitcode}, '\
                         f'got {x["exitcode"]}\n{self.dump_stat(x)}'