[media] dvb-frontends: fix potential NULL pointer dereference in stv0900_set_mclk()

The dereference should be moved below the NULL test.
dpatch engine is used to auto generate this patch.
(https://github.com/weiyj/dpatch

)

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>