quagga: fix segment fault when stopping ospf6d

In ospf6_clean, the variable ospf6 might be NULL causing segment fault
when stopping ospf6d. Check the variable before referencing it.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Joe MacDonald <joe_macdonald@mentor.com>