http2: fix potential warning about narrowing

By passing both arguments as qint64, then casting to qint32.
Since one of the arguments are qint32, the result will fit in qint32.

Change-Id: I98e9b1484549fa5dad340f02eda1b341cf6b293d
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Ahmad Samir <a.samirh78@gmail.com>
Reviewed-by: Konrad Kujawa <konrad.kujawa@qt.io>

src/network/access/qhttp2protocolhandler.cpp

@@ -504,7 +504,7 @@ bool QHttp2ProtocolHandler::sendDATA(Stream &stream)
         }
 
         frameWriter.start(FrameType::DATA, FrameFlag::EMPTY, stream.streamID);
-        const qint32 bytesWritten = std::min<qint32>(slot, chunkSize);
+        const qint32 bytesWritten = qint32(std::min<qint64>(slot, chunkSize));
 
         if (!frameWriter.writeDATA(*m_socket, maxFrameSize, src, bytesWritten))
             return false;