V4L/DVB: TM6000: Fix code which cause memory corruption

The driver was doing malloc when buf is null causing memory corruption.

The analog part is still pretty much broken but at least fixing this
will stop it from crashing the machine when streamon.

Signed-off-by: Bee Hock Goh <beehock@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>