ralloc: don't write to memory in case of alloc fail.

ralloc: don't write to memory in case of alloc fail.

For some reason I made this happen under indirect rendering,
I think we might have a leak, valgrind gave out, so I said I'd
fix the basic problem.

NOTE: This is a candidate for stable branches.
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>