qv4l2: fix a coverity defect

qv4l2: fix a coverity defect

*** CID 1226677:  Unused pointer value  (UNUSED_VALUE)
/utils/qv4l2/ctrl-tab.cpp: 64 in ApplicationWindow::addWidget(QGridLayout *, QWidget *, QFlags<Qt::AlignmentFlag>)()
58     	}
59     }
60
61     void ApplicationWindow::addWidget(QGridLayout *grid, QWidget *w, Qt::Alignment align)
62     {
63     	QToolButton *tb;
>>>     CID 1226677:  Unused pointer value  (UNUSED_VALUE)
>>>     Pointer "tb" returned by "qobject_cast(w)" is never used.
64     	if (m_col % 2 && !(tb = qobject_cast<QToolButton*>(w)))
65     		w->setMinimumWidth(m_minWidth);
66     	if (w->sizeHint().width() > m_maxw[m_col])
67     		m_maxw[m_col] = w->sizeHint().width();
68     	if (w->sizeHint().height() > m_maxh)
69     		m_maxh = w->sizeHint().height();

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>