[media] V4L: sh_mobile_ceu_camera: fix Oops when USERPTR mapping fails

If vb2_dma_contig_get_userptr() fails on a videobuffer, driver's
.buf_init() method will not be called and the list will not be
initialised. Trying to remove an uninitialised element from a list leads
to a NULL-dereference.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Tested-by: Bastian Hecht <hechtb@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>