drm/radeon/kms: Fix NULL pointer dereference if memory allocation failed.

When there is allocation failure in radeon_cs_parser_relocs parser->nrelocs
is not cleaned. This causes NULL pointer defeference in radeon_cs_parser_fini
when clean up code is trying to loop over the relocation array and free the
objects.

Fix adds a check for a possible NULL pointer in clean up code.

Signed-off-by: Pauli Nieminen <suokkos@gmail.com>
Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>