Don't crash if Clipboard and Selection hold the same mimedate

Cliboard and Selection can hold the same QMimeData object.
Don't delete the mimedata in this case before when clearing
only one of them.

Reviewed-by: Samuel

src/plugins/platforms/xcb/qxcbclipboard.cpp

@@ -272,7 +272,8 @@ void QXcbClipboard::setMimeData(QMimeData *data, QClipboard::Mode mode)
 
     xcb_window_t newOwner = XCB_NONE;
 
-    delete m_clientClipboard[mode];
+    if (m_clientClipboard[QClipboard::Clipboard] != m_clientClipboard[QClipboard::Selection])
+        delete m_clientClipboard[mode];
     m_clientClipboard[mode] = 0;
     m_timestamp[mode] = XCB_CURRENT_TIME;