media: rcar-vin: fix null pointer dereference in rvin_group_get()

Store the group pointer before disassociating the VIN from the group.

Fixes: 3bb4c3bc ("media: rcar-vin: add group allocator functions")

Reported-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>