drm: fix blob pointer check

Check properly that the allocated blob's pointer is valid.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Daniel Stone <daniels@collabora.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: dri-devel@lists.freedesktop.org
Reviewed-by: Daniel Stone <daniels@collabora.com>
Fixes: 5488dc16 ("drm: introduce pipe color correction properties")
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1457611461-9116-1-git-send-email-lionel.g.landwerlin@intel.com