security: avoid calling a NULL function pointer in drivers/video/tvaudio.c

security: avoid calling a NULL function pointer in drivers/video/tvaudio.c

NULL function pointers are very bad security wise. This one got caught by
kerneloops.org quite a few times, so it's happening in the field....

Fix is simple, check the function pointer for NULL, like 6 other places
in the same function are already doing.

Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
[2.6 commit: 5ba2f67afb02c5302b2898949ed6fc3b3d37dcf1]
Signed-off-by: Willy Tarreau <w@1wt.eu>