backport: fix leak when creating wxCURSOR_BLANK

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_2_8_BRANCH@51000 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/gtk/cursor.cpp

@@ -74,6 +74,7 @@ wxCursor::wxCursor( int cursorId )
                                                                     &color,
                                                                     &color,
                                                                     0, 0);
+                g_object_unref(pixmap);
             }
             return;