http2: discard frames with no SessionHandle

Return 0 instead of NGHTTP2_ERR_CALLBACK_FAILURE if we can't locate the
SessionHandle. Apparently mod_h2 will sometimes send a frame for a
stream_id we're finished with.

Use nghttp2_session_get_stream_user_data and
nghttp2_session_set_stream_user_data to identify SessionHandles instead
of a hash.

Closes #372

lib/http.c

@@ -1443,7 +1443,10 @@ CURLcode Curl_http_done(struct connectdata *conn,
                         CURLcode status, bool premature)
 {
   struct SessionHandle *data = conn->data;
-  struct HTTP *http =data->req.protop;
+  struct HTTP *http = data->req.protop;
+#ifdef USE_NGHTTP2
+  struct http_conn *httpc = &conn->proto.httpc;
+#endif
 
   Curl_unencode_cleanup(conn);
 
@@ -1482,6 +1485,10 @@ CURLcode Curl_http_done(struct connectdata *conn,
     free(http->push_headers);
     http->push_headers = NULL;
   }
+  if(http->stream_id) {
+    nghttp2_session_set_stream_user_data(httpc->h2, http->stream_id, 0);
+    http->stream_id = 0;
+  }
 #endif
 
   if(HTTPREQ_POST_FORM == data->set.httpreq) {

lib/http.h

@@ -215,7 +215,6 @@ struct http_conn {
                               nghttp2_session_mem_recv */
 
   /* this is a hash of all individual streams (SessionHandle structs) */
-  struct curl_hash streamsh;
   struct h2settings settings;
 #else
   int unused; /* prevent a compiler warning */