[PATCH] dvb: dib3000: add NULL pointer check

prevent NULL pointer related Oopses (Patrick Boettcher)

Signed-off-by: Johannes Stezenbach <js@linuxtv.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>