Fix memory leak of wxClipboard data on exit

When wxClipboard is destroyed as part of the program shutdown,
gdk_selection_owner_get() doesn't return our clipboard widget as owner
any more, so we don't reset the owner when Clear() is called and hence
never free the data.

Do it explicitly if we don't have clipboard ownership in Clear() any
longer to avoid memory leaks -- even though they are mostly harmless (as
they happen only once, on exit), they still show up in LSAN and similar
tools reports.

src/gtk/clipbrd.cpp

@@ -587,6 +587,15 @@ void wxClipboard::Clear()
         // it will free our data
         SetSelectionOwner(false);
     }
+    else
+    {
+        // We need to free our data directly to avoid leaking memory.
+        delete m_dataPrimary;
+        m_dataPrimary = nullptr;
+
+        delete m_dataClipboard;
+        m_dataClipboard = nullptr;
+    }
 
     m_targetRequested = nullptr;
     m_formatSupported = false;