HTTP/2 - fix invalid read (auto-test)

Since headersFrame is a reference to a vector's element, clearing this
vector before accessing headersFrame.flags is not a good idea, must be
done later.

Change-Id: I80eee0761ac1cad580e979be9371ec7588a694ac
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

tests/auto/network/access/http2/http2srv.cpp

@@ -625,12 +625,12 @@ void Http2Server::processRequest()
         return;
     }
 
-    continuedRequest.clear();
     // Actually, if needed, we can do a comparison here.
     activeRequests[streamID] = decoder.decodedHeader();
     if (headersFrame.flags.testFlag(FrameFlag::END_STREAM))
         emit receivedRequest(streamID);
     // else - we're waiting for incoming DATA frames ...
+    continuedRequest.clear();
 }
 
 QT_END_NAMESPACE