commit 2420489b upstream.

On the DMA mapping error path, sg may be NULL (it has already been
marked as the last scatterlist entry), and we should avoid dereferencing
it again.
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: e2273302 ("drm/i915: avoid leaking DMA mappings")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Imre Deak <imre.deak@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161114112930.2033-1-chris@chris-wilson.co.ukReviewed-by: Matthew Auld <matthew.auld@intel.com>
(cherry picked from commit b17993b7)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>