V4L/DVB (8100): V4L/vivi: fix possible memory leak in vivi_fillbuff

Move allocation after first check and fix memory leak.

Noticed-by: Daniel Marjamäki <danielm77@spray.se>

Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>