V4L/DVB (10304): buf-dma-contig: fix USERPTR free handling

This patch fixes a free-without-alloc bug for V4L2_MEMORY_USERPTR
video buffers.

Signed-off-by: Magnus Damm <damm@igel.co.jp>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>