QTextStream: discard+comment the possibility of file opening failure

The QTextStream(FILE*) constructor may fail to open the filehandle
through QFile (e.g. if the open mode is incompatible). However
QTextStream has no error report mechanism for this and still reports
status Ok. This is consistent for instance with the QIODevice*
constructor, where even passing a closed device sets the status to Ok.
Add a comment and discard the return of open().

Change-Id: I430b96fd26e0ebca15a4d9ee640b09895bdd0b03
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/serialization/qtextstream.cpp

@@ -1002,7 +1002,10 @@ QTextStream::QTextStream(FILE *fileHandle, OpenMode openMode)
            fileHandle, int(openMode));
 #endif
     QFile *file = new QFile;
-    file->open(fileHandle, openMode);
+    // Discarding the return value of open; even if it failed
+    // (and the file is not open), QTextStream still reports `Ok`
+    // for closed QIODevices, so there's nothing really to do here.
+    (void)file->open(fileHandle, openMode);
 
     Q_D(QTextStream);
     d->device = file;