Fix fatal error on unsupported board size

Fix fatal error on unsupported board size

When a board size unsupported by the engine was requested through the
New Variant dialog, this was discovered while the dialog was still up,
causing the error message to pop up as a child of the New Variant dialog,
so that it would immediately be taken down as the latter closed.
As a result it could not be confirmed, so that XBoard would not exit, and
was left in a pathologic state (ignoring window-close). This is fixed
by performing Reset (which detects the error) after PopDown of New Variant.