[QTextStream::read.*] Return earlier when nothing has been read

There is no any sense in detecting the encoding of an empty string ;)

Change-Id: I1c7af07bd7c3e7e7cf67421a2cb3a1123ca57650
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>

src/corelib/io/qtextstream.cpp

@@ -447,6 +447,9 @@ bool QTextStreamPrivate::fillReadBuffer(qint64 maxBytes)
             bytesRead = device->read(buf, sizeof(buf));
     }
 
+    if (bytesRead <= 0)
+        return false;
+
 #ifndef QT_NO_TEXTCODEC
     // codec auto detection, explicitly defaults to locale encoding if the
     // codec has been set to 0.
@@ -470,9 +473,6 @@ bool QTextStreamPrivate::fillReadBuffer(qint64 maxBytes)
            qt_prettyDebug(buf, qMin(32,int(bytesRead)) , int(bytesRead)).constData(), int(sizeof(buf)), int(bytesRead));
 #endif
 
-    if (bytesRead <= 0)
-        return false;
-
     int oldReadBufferSize = readBuffer.size();
 #ifndef QT_NO_TEXTCODEC
     // convert to unicode