QNonContiguousByteDeviceThreadForwardImpl: reset m_atEnd during reset()

Because otherwise the atEnd() function will return true if we finished
an upload, and then we reset due to an error and try to upload again.
In a future patch we rely on atEnd() returning true only if the data
has actually been consumed, so re-uploads will not work correctly.

Pick-to: 6.6 6.5
Change-Id: Id914648f2509dd112b6bfc71231caa59cbe8ae68
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

src/network/access/qhttpthreaddelegate_p.h

@@ -248,6 +248,7 @@ public:
         if (b) {
             // the reset succeeded, we're at pos 0 again
             m_pos = 0;
+            m_atEnd = false;
             // the HTTP code will anyway abort the request if !b.
         }
         return b;