media: dvb-core: add missing buffer index check

dvb_vb2_expbuf() didn't check if the given buffer index was
for a valid buffer. Add this check.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Reported-by: Chenyuan Yang <chenyuan0y@gmail.com>
Closes: https://lore.kernel.org/linux-media/?q=WARNING+in+vb2_core_reqbufs


Fixes: 7dc866df ("media: dvb-core: Use vb2_get_buffer() instead of directly access to buffers array")
Reviewed-by: Benjamin Gaignard <benjamin.gaignard@collabora.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>