media: dvb-core: Use vb2_get_buffer() instead of directly access to buffers array

Use vb2_get_buffer() instead of directly access to vb2_buffer buffer array.
This could allow to change the type bufs[] field of vb2_buffer structure if
needed.
After each call to vb2_get_buffer() we need to be sure that we get
a valid pointer so check the return value of all of them.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@collabora.com>