V4L/DVB (13685): Correct code taking the size of a pointer

sizeof(print_buf) is just the size of the pointer.  Change it to the size
used in the allocation of print_buf earlier in the same function.

A simplified version of the semantic patch that finds this problem is as
follows: (http://coccinelle.lip6.fr/

)

// <smpl>
@@
expression *x;
expression f;
type T;
@@

*f(...,(T)x,...)
// </smpl>

Cc: Janne Grunau <j@jannau.net>
Signed-off-by: Julia Lawall <julia@diku.dk>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>